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Shell Morphology
Systematic Paleontology
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Systematic Paleontology, continued

Family Euchondriidae Newell, n. fam.

Aviculopectinoids in which, in addition to a relatively large, oblique median resilifer, a series of narrow quadrate and transverse resilifers occurs in front and behind the beaks, extending along flat and diverging ligament areas comparable to those in Aviculopecten; the outer ostracum of the right valve in the type genus is prismatic, that of the left structureless.

The general form, shell structure, and forward obliquity of the median resilifer in Euchondria suggests that it was derived from the Aviculopectininae. The lack of radial costae on the right valve might be interpreted to mean that the genus is an offshoot from a very primitive member of this subfamily. The family Euchondriidae includes Euchondria and possibly Crenipecten.

The hypothetical phylogeny of the Euchondriidae is indicated in figure 17.

Genus EUCHONDRIA Meek, 1874

Euchondria MEEK, 1874, Am. Jour. Sci., 3 ser., vol. 7, pp. 445, 488.

Markedly prosocline shells with a more or less prominent extension at the posteroventral margin; left valve ornamented with relatively widely spaced intercalate costae, which may be lacking on the auricles and obscure on the shell body; costae crossed by regular distinct fila which may be as coarse and widely spaced as the costae, or relatively fine and closely spaced; right valve smooth or with obscure concentric fila; costae, when present, only on auricles; distinct umbonal folds lacking in left valve, obscure in right valve; auricle sulci broad; hinge about one half as long as the shell, extremities more or less quadrate; inner ostracum composed of concentric crossed lamellar aragonite, outer ostracum prismatic calcite in the right valve, apparently structureless calcite in the left; prisms commonly square in cross-section, uniform in size, and arranged in radial and concentric rows; ligament area as in Aviculopecten but having in addition a series of rectangular resilifers transverse to the hinge length in front and behind the larger median resilifer, the resilifers increasing in size and spacing toward the extremities of the hinge; right valve only slightly less convex than the left; musculature similar to that of Aviculopecten; there is a tendency toward development of auricular crura, the posterior crus being about three times as long as the anterior. There is a marked anterior and posterior gape of the valves.

Measurements in Species of Euchondria
Species	Specimen No.	Length, mm	Height, mm	Hinge length, mm	Umbonal angle, degrees	No. auricular costae		Body costae at given height
						Anterior	Posterior	Costae	Height
levicula	L1	7	7	5	95	3	3+	33	7
	L2	6.5	6	4.5	95	3	4	34	6
	L3	12	11	7±	93	3	5+	44	11
	L4	11	10	6.5	95	3	3	48	10
	L5	9	7.5	5	100±	3	3	47	7.5
	L6 a	10	9	5.5	93	3	4	47	9
	R7	7.5	7.5	6	94		2
	R8	7	7	5.5	91
	R9	9	9	6.5	105	2±	4
subcancellata	L1 a	6	5.5	3	90
	L2		6	3	90		4		4
pellucida	L1 a	9	8.6	6.5	94	2-3	4	47	8.6
	L2	10	9	6-7	90±	2		55	9
ohioensis	L1 a	4	4	2.4	77
	L2	4.8	5	2.8	85±
smithwickensis	L1	34	33	21.5	98	2-3	5±	64	33
	L2	34	33±		95-100			58	33
	R3	26.5	26.5	21.5	110				26.5
	R4	25	26	21.5	105				26
a. Holotype.

Genotype—Pecten neglectus Geinitz, Table Creek shale, Pennsylvanian, Nebraska City, Neb.

Range—Devonian? to Upper Carboniferous.

Remarks—There has been a general misconception regarding the ornamentation in Euchondria. Since Meek's diagnosis, it has been assumed that both valves, excepting the auricles, are noncostate. Indeed, this idea appears to date from the original description of Pecten neglectus Geinitz. The holotype of the species is a smooth shell, described as a left valve. Geinitz apparently had only the one specimen. When Meek restudied the fauna at Nebraska City (Meek, 1872) he was able to recognize only right valves of the species, and subsequent descriptions of the species have been based on right valves only.

In the collections that I have studied there are some extraordinarily fine specimens of Euchondria from the St. Louis Pennsylvanian outlier, collected by Dr. J. Brookes Knight. These specimens are free valves showing the original shell structure, color patterns, and hinge characters. Each left valve bears more or less distinct costae crossed by fila, so that there is a reticulate pattern. The right valves are like the right valves of E. neglecta from Nebraska City, figured by Meek (1872, pl. 9, fig. 1b.). A large collection which I have from Winterset, Iowa, contains scores of specimens of a small pectinoid, right and left valves being represented in about equal numbers. The specimens are preserved in shale, and only the outer ostracum remains. The left valves are costate and the right ones are smooth. This relation is seen again in another species from the Smithwick shale (Morrow subseries, Texas), in which the hinge characters of the genus are admirably displayed. It appears, then, that the Euchondria hinge occurs in shells having a costate left valve. It will be recalled that the so-called Crenipecten winchelli (Meek), from the Mississippian Waverly sandstone in Ohio, has a smooth right valve and a costate left one. In all of these forms, perhaps excepting that from the Waverly which I have not seen, the outer prismatic ostracum of the right valve has a very striking arrangement of the prisms. The individual prisms are quadrate in cross section, except for irregular ones near the margin in full sized specimens, and they are regularly arranged in radial and concentric rows.

Examination of Geinitz's illustration of the type of Pecten neglectus indicates strong probability that he figured a right valve in which the anterior auricle is broken or partly covered by matrix, rather than a left valve, as stated. In addition to the smooth surface of his type, it will be noted that the umbonal fold on the left hand margin of his shell is longer than that on the right hand margin. My observations indicate uniformly that the posterior umbonal fold is longer than the anterior in these shells. Furthermore, in the collections at hand, the two complementary valves of Euchondria are not exactly the same in shape, the right valves being more nearly symmetrical and less strongly prosocline than the left. The type of Pecten neglectus, as figured by Geinitz, and Meek's topotype right valve agree rather closely in the slight obliquity, rather than differing markedly as they should if they were complementary instead of both being right valves. It is not without significance that Meek did not recognize left valves of Pecten neglectus in his own collections from Geinitz's locality. Summing up the evidence, it seems that left valves of Euchondria have not been recognized as such in our American Carboniferous rocks because of the erroneous assumption that the two valves are alike in ornamentation.

In the absence of topotypes of left valves of E. neglecta (Geinitz) I find it difficult to recognize the species. Meek (1872, pl. 9, figs. 2a b) figured a specimen from Nebraska City under the name Aviculopecten coxanus, which it certainly is not. It appears probable to me that this figured specimen is a left valve of E. neglecta, for it has all of the general external characters of the left valves of other euchondrias in our collections, such as marked obliquity, acute auricles, poorly developed umbonal folds, and small size. A rather marked development of fila is observed on the shell, but these are much finer than is common in Euchondria.

The resemblance between left valves of Euchondria and Aviculopecten s. s. is really remarkable when the difference in general organization is appreciated. Such forms as E. smithwickensis, n. sp., and E. winchelli (Meek) can be recognized generically only with difficulty from external features of isolated left valves. The characters of the hinge, lack of umbonal folds, and the marked shell gape are generally sufficient, however, to distinguish between left valves of the two genera, and commonly the concentric fila are sufficiently coarse to produce a reticulate pattern not displayed by many typical aviculopectens.

It is worthy of note that most of our specimens of Euchondria occur in black carbonaceous shale or limestone, and argillaceous gray shale. In no instance has a specimen been found in gray or white limestone.

Euchondria was a long-lived genus, being typically represented in Early Mississippian beds, and possibly also by such Late Devonian species as Crenipecten liratus Hall.

EUCHONDRIA NEGLECTA (Geinitz)

Plate 19, figures 1, 4

Pecten neglectus Geinitz, 1866, Carbonformation und Dyas in Nebraska, p. 33, pl. 2, fig. 17.
Aviculopecten neglectus Meek, 1872, U. S. Geol. Survey of Nebraska, p. 193, pl. 9, figs. 1a, b.
Aviculopecten coxanus Meek, 1872, U. S. Geol. Survey of Nebraska, p. 196, pl. 9, figs. 2a, b.

Meek's description:

Shell very small, broad subovate, exclusive of the ears, very thin, rather compressed; sides and base more or less regularly rounded; cardinal margin shorter than the breadth of the valves. Left valve (according to Professor Geinitz's figure) with ears nearly equal, the anterior one nearly separated from the margin below by a broad, very shallow sinus, and forming less than a right angle at its extremity; posterior ear extending farther down the margin than the other, very faintly sinuous behind. and forming an angle of about 100° at the extremity. Right valve with anterior ear narrow and rather acutely angular, defined by a deep, narrow sinus, extending back about half its length; posterior ear of about the same length. but of greater vertical breadth than the other, rather pointed at the extremity, and defined by a moderately deep, broadly rounded sinus, and a subangular umbonal slope. Surface of the body part of both valves apparently only marked by fine concentric striae; ears with a few radiating costae, crossed by fine striae and a few coarser marks of growth.

Height and breadth each, 0.26 inch; length of hinge, 0.21 inch.

This little species is rather remarkable in having the body part of the valves with apparently only fine concentric striae, while the ears are ornamented with a few comparatively distinct radiating costae. I know of no species with which it is liable to be confounded.

The specimen figured by Professor Geinitz is a left valve, while those I have seen are all right valves. The latter are mainly casts, but one of them retains portions of the shell. On raising a small piece of this with the point of a knife. and placing it under the misroscope, where it could be examined by a strong transmitted light, it was found to present distinct indications of a prismatic structure, apparently not due to crystallization. As this, the only remaining portion of the shell, is exceedingly thin. and consists of a single apparently prismatic layer, I have little doubt that the inner laminated portion of the shell has been dissolved away, as seems to have been the case in other species in these rocks.

It has been indicated already in discussion of the genus Euchondria that it seems highly probable that Meek's Aviculopecten coxanus from Nebraska City is a left valve of E. neglecta. The most obvious distinction between Meek's A. coxanus and other species of Euchondria, is the small size and close spacing of the fila. In our collections there is a Euchondria from the Deer Creek formation having the same characters of ornamentation as the Nebraska City coxanus. In one of our specimens the hinge characters of Euchondria are shown. Meek's description of the Nebraska City specimen, called Aviculopecten coxanus by him, is as follows:

Shell very small; thin, compressed. slightly oblique; broad subovate, exclusive of ears; basal margin, rounded; anterior margin more or less rounded, rather straight and oblique; posterior margin more prominent than the anterior, often subangular at the point where the posteriorbasal margin rounds up to meet the obliquely-sloping edge above. Hinge generally a little less than the greatest breadth of the valves below. Left valve with anterior ear of moderate size, flat triangular, with the extremity generally a little less than a right angle, sometimes very slightly rounded, separated from the margin below by an abruptly rounded or subangular sinus; posterior ear slightly larger and much more acutely angular than the other, but shorter than the most prominent part of the margin below, from which it is separated by a moderately deep rather broadly rounded sinus; beak small, compressed, scarcely projecting beyond the cardinal margin, and placed a little in advance of the middle of the hinge; surface ornamented with linear, simple, often more or less flexuous costae, which alternate in size, the smaller ones dying out at the various distances between the free margins and the umbo-crossing; all of these are numerous, extremely fine, regular, closely arranged concentric striae, which, like the costae, are more or less distinctly defined on the ears, as well as on the body of the valve. Right valve unknown.

Height of left valve, 0.37 inch; breadth of do. 0.39 inch; convexity about 0.05 inch.

Comparison—It seems impossible to distinguish E. neglecta from other small species of the genus solely on the basis of the right valve. If the form described as Aviculopecten coxanus from Nebraska City by Meek is a left valve of Euchondria neglecta, as I now believe, then the species is particularly characterized by the closely spaced and unusually fine fila. In this respect it is like E. smithwickensis. The latter is a very much larger shell, however, and is nearly or quite lacking in auricular costae. There appears to be a fairly well developed cardinal costa on each side of the beak in left valves, at least, of the present form.

Material—Unfortunately, I have not yet been able to examine the types or topotypes from Nebraska City, and the other material at present referred to this species is sparse and poorly preserved. Found in gray argillaceous and dark-gray to black fissile, carbonaceous shale.

Occurrence—Pennsylvanian series (Upper Carboniferous). The types of E. neglecta came from the Table Creek shale (Wabaunsee group, Virgil subseries) at Nebraska City, Neb. Other specimens, left valves, now referred to the species, were found in the Jones Point shale (Shawnee group, Virgil subseries) one half mile southeast of Wabash, Neb.; and Ervine Creek limestone (Shawnee), Stout quarry, Haynies, Iowa.

EUCHONDRIA PELLUCIDA (Meek and Worthen)

Plate 19, figures 16, 17, 19

Aviculopecten pellucidus Meek and Worthen, 1860, Philadelphia Acad. Nat. Sci., Proc., p. 435; 1866, Illinois Geol. Survey, vol. 2, p. 327, pl. 26, figs. 5a, b.

This species is a typical small Euchondria, characterized by having a somewhat deeper anterior auricular sulcus, more rounded and less produced posterior extension of the shell body, and a very angular anterior auricle with a straight front margin. The surface of the left valve is cancellated with fine costae, crossed by almost equally coarse and similarly spaced fila. There are moderately well-developed cardinal costae.

On the same shale fragment with the types occurs an imprint of a posterodorsal fragment of a right valve, apparently until now quite overlooked. This impression shows the characteristic alignment of prisms of the outer ostracum in concentric rows, and shows six costellae on the rear auricle, besides a larger cardinal costa. There are about 45 or 50 of the concentric rows of prisms in a space of 1 mm.

Measurements for specimens of Euchondria pellucida are given in the table previous.

Comparison—The distinguishing features of the species have been indicated. It appears that the number of costae in this species is somewhat higher than in comparable forms.

Material—Through the generosity of Dr. J. Marvin Weller, I have been able to study and illustrate the type specimens of the species. There are two left valves and a fragment of a right valve (Univ. Ill., No. 10,932) which are poorly preserved as impressions in shale of the inner surface of the outer ostracum. The smaller and more complete of the two left valves is here designated the holotype. Found in black, fissile, carbonaceous shale.

Occurrence—Pennsylvanian (Upper Carboniferous), Des Moines beds, Adams county, Illinois. A label accompanying the types bears the notation "L 7 p e." In reference to the age of the beds exposed in Adams county, Dr. Harold Wanless writes: "The marine section in Adams county, Illinois, includes several horizons. The highest would be slightly below your Pawnee limestone and the lowest would probably be above your Tebo coal." The species has not yet been found elsewhere.

EUCHONDRIA SMITHWICKENSIS Newell, n. sp.

Plate 19, figures 12-13b

This is a very large Euchondria, in which the concentric fila are relatively very fine and closely spaced, so that the general aspect of the left valve, with its prosocline obliquity and slender costae, is remarkably like that of some of the Devonian pseudaviculopectens. The auricles are acute, the anterior one of both valves having a nearly straight anterior margin. Cardinal margin of the left valve nearly two thirds as long as the shell, and that of the right valve nearly four fifths as long as the shell body of the same valve. Obviously, this relation indicates that the right valve was proportionally smaller than the left as in Pennsylvanian species of Aviculopecten. As in other species of the genus there is no posterior umbonal fold in the left valve, the umbonal convexity sloping outward uniformly to the auricle without any abrupt change in convexity, but there is a distinct narrow anterior fold. A marked shell gape occurs at the front and rear ends. The umbonal folds of the right valve are moderately distinct and straight. One right valve shows well the generic characters of the hinge and shell structure. The auricles of the left valve are obscurely costate; those of the right valve are smooth and there are no cardinal costae.

The left valves in my possession are external molds, so that it is through inference that I class the two very dissimilar valves as belonging to the same genus and species. If it were not for independent proof that unlike valves such as these are the rule in Euchondria, it would seem absurd to consider them as complementary valves of one species. I have two slabs of rock, each bearing one right and one left valve on the same bedding surface. I have always considered this kind of association important supplementary evidence of the equivalency of unlike valves.

Meek (1875, pp. 296-298) described a similar association of unlike right and left valves in his Aviculopecten winchelli from the Waverly beds at Newark, Ohio. He recognized a close similarity, except in size, between this species and the one that he had described as A. coxanus from Nebraska City. Hall (1884, p. 89) described the hinge of A. winchelli as crenulate, and classed the species as Crenipecten. It now seems nearly certain that the species is a Euchondria. At any rate the present species, E. smithwickensis, is remarkably like E. winchelli.

Measurements of specimens of E. smithwickensis are given in the table previous.

Comparison—This form is comparable only to the Mississippian E. winchelli (Meek), but it is different from that species in the angularity of the auricles and in lacking costae on the auricles of the right valve.

Material—Holotype and three topoparatypes Yale Univ., Peabody Mus., Nos. 8,103, 14,479. The left valves are preserved as external molds and the right valves are more or less ent.ire, exhibiting the original structures of the shell. Found in black, carbonaceous, silty limestone.

Occurrence—Pennsylvanian (Upper Carboniferous). Smithwick shale (Bend group, Morrow subseries). The holotype and one topoparatype bear the label "east of San Saba, Texas." The two other topoparatypes are from the same locality.

EUCHONDRIA SUBCANCELLATA Newell, n. sp.

Plate 19, figures 2, 3, 6, 7

Typical small euchondrias esepecially characterized by an obsolete or obsolescent condition of the costae over the middle and anterior parts of the left valve. The fila, characteristic of the genus, are faint and closely spaced. Although the ornamentation is similar to that in E. menardi (Worthen) the front margin of the anterior auricle, unlike that of E. menardi is broadly rounded.

Measurements of specimens of E. subcancellata are given in the table previous.

Comparison—The particular characters by which this species can be recognized have been stated above. Several of the specimens are somewhat corroded and it was at first thought that these were worn specimens of E. levicula, n. sp., with which they are associated. The fila are more closely spaced than in E. levicula, however, and in several instances extend across the middle of the shell where no costae are visible.

Material—Several scores of specimens almost all of them exquisitely preserved, however, with the surface more or less corroded (Kansas Univ., Paleonto Type Coll., No. 396).

Occurrence—Pennsylvanian (Upper Carboniferous). Upper Labette shale, Des Moines subseries, St. Louis, Mo., Knight's locality 43.

EUCHONDRIA LEVICULA Newell, n. sp.

Plate 1, figures 6, 7; Plate 19, figures 5, 10, 11, 18

This is a small characteristic Euchondria having a particularly distinctive ornamentation of the left valve. The costellae and concentric fila are prominent, but quite slender and uniform. The fila are spaced about the same distance apart as the costellae, so that the surface is regularly cancellate in nearly equidimensional rectangles. The interval between fila is about 0.4 mm. across the median part of the shell and does not change appreciably across the adult part of the shell, except in the margin of gerontic individuals where the fila become crowded and less regular. Although the general appearance of the shell is like that of E. pellucida (Meek and Worthen), there are slightly fewer costellae. From the latter species E. levicula is readily distinguished by having the anterior margin of the front left auricle broadly curved instead of straight, and the posterior margin of the shell in E. levicula is broadly concave rather than nearly straight as in E. pellucida. There are obscure cardinal costae on both valves.

The species is based on a large number of remarkably uniform specimens in shale, from a single locality in the Bronson group. Other extraordinarily well preserved specimens showing characters of shell structure, articulus, and musculature are contained in the fine collection of pelecypods from the St. Louis outlier (Marmaton) given me by Dr. J. Brookes Knight. There is somewhat greater variation in these specimens than in the larger collection from the Bronson group, and for that reason it has seemed desirable to designate one of the Bronson specimens as the holotype even though the specimen is preserved only as an external mold.

Measurements for specimens of E. levicula are given in the table previous.

Comparison—This species is like E. pellucida; the means by which the two are distinguished have been indicated in the above description.

Material—There are several scores of specimens from one locality, including the holotype, at Yale Univ., Peabody Mus., No. 14,478. In these specimens only the outer ostracum is present, but. the ornamentation is perfectly preserved. In addition there are several fine specimens (Kansas Univ., Paleont. Type Coll., No. 397) from the unique Knight collection of St. Louis outlier fossils. Many of these specimens retain not only the original shell structure and composition, but also the original color pattern in the form of broad irregular radial bands. Found in black fissile shale, argillaceous shale, and variegated maroon and green calcareous shale.

Occurrence—Pennsylvanian (Upper Carboniferous). The holotype and topoparatypes were found in the Hushpuckney black and gray shale, Swope formation (Missouri subseries), at Devil's Backbone, near Winterset, Iowa. The specimens in the Knight collection are from near the top of the Labette shale, Knight's loco 43, near St. Louis, Mo. The species has not been recognized elsewhere.

EUCHONDRIA OHIOENSIS (Mark)

Plate 19, figures 8, 9

Acanthopecten ohioensis Mark, 1912, Ohio Geol. Survey, Bull. 17, 4th ser., p. 308, pl. 15, fig. 5.

The type specimens before me are not sufficient for the recognition of this species, but the original locality and horizon are known and it therefore eventually will be possible to learn more about the species.

The shells are markedly prosocline, small, with well-defined, though small auricles. The posterior margin of the shell body is notably concave, meeting the extremity of the ventral margin at a rounded right angle. The front margin of the shell body is nearly straight. The surface of the specimens is very badly worn, but still there is a trace of fine reticulate ornamentation produced by interesting costellae and fila. In one specimen the costellae and fila are about equally spaced at somewhere near 70 microns. There is no evidence of ornamentation on the auricles, but the preservation is so poor that this cannot be determined certainly. The shell substance shows clearly concentric crossed-lamellar structure. Apparently the outer ostracum is nearly or entirely lost by abrasion. The anterior auricle is broadly rounded at the front margin.

Measurements of specimens of E. ohioensis are given in the table previous.

Comparison—Beyond the probability that this form is a Euchondria, it can scarcely be recognized because of the exceedingly poor preservation of the type specimens.

Material—Through the kindness of Prof. J. E. Carman, I have been enabled to examine the types of Mark's Acanthopecten ohioensis. The types are two poorly preserved left valves, both probably juveniles. The shell substance is so soft and friable that the outer surface has suffered considerable abrasion and the original ornamentation almost obliterated. The specimens do not resemble the original figure, in spite of the strong supposition that it was drawn from one of the specimens before me. An extended search for further type specimens at Ohio State University has failed. It may be assumed that the holotype is lost, or that the original figure is a gross misrepresentation of the specimen before me, supposed to be the holotype. At any rate, it appears likely that further topotypes can be used to satisfactorily establish the species. Found in black, carbonaceous, fine-grained limestone.

Occurrence—Pennsylvanian (Upper Carboniferous). Portersville limestone (Conemaugh), Portersville, Ohio.

EUCHONDRIA MENARDI (Worthen)

Plate 19, figures 14a-15b

Lima? menardi Worthen, 1884, Illinois State Mus. Nat. History, Bull. 2, p. 22.
Aviculopecten menardi Worthen, 1890, Illinois Geol. Survey, vol. 8, p. 120, pl. 22, fig. 12.

This species was founded on a poorly preserved specimen and topotypes are needed for a proper evaluation of characters shown by the holotype.

The holotype is a crushed and completely flattened left valve exposing the inner surface of the thin outer ostracum. Where fiakes of the ostracum layer have fallen away from the matrix, an external mold remains, so that something can be seen of the outer ornamentation. On the same shale fragment is a small, poorly preserved imprint of a right valve, possibly the complement to the holotype, showing something of the Euchondria ligament area. This specimen, overlooked by Worthen, appears to be an internal mold.

There are traces of obscure costae on the rear auricle and posterior part of the shell, but they are too faint to be counted accurately through the shell layer. In front of a place on the margin directly under the beak, there are 11 irregular and obscure costae. The posterior half of the shell appears to be devoid of any costae. Two distinct, narrow costae cross the middle part of the posterior auricle and there are traces of one or two more on each side of them, making a total of five or six. All of the auricular costae die out before reaching the margin, suggesting that the shell had reached a geronic stage. Cardinal costae absent. The shell is ornamented by somewhat irregular distinct fila, spaced on mature areas a maximum of 130 microns on the auricles and about 120 microns on the median part of the shell body. The median part of the dorsal two thirds of the body of the shell, including the beak is practically devoid of any ornamentation, even the fila being confined to the auricles and shell margin. It is not possible to determine whether or not the shell was abraded or corroded before fossilization, but it seems likely that the shell has been altered in some such fashion. The byssal sinus in both valves shows a marked reduction during shell growth until the left anterior auricle has a nearly straight margin, and the notch of the right valve reduced to a sinus. Such changes during growth are probably to be correlated with a partial or complete loss of the function of the foot. The posterior auricle of the right valve has three or four faint costellae across the middle third of the auricle. Fila like those on the left valve ornament the surface of the right posterior auricle, but the anterior one has only fine, irregular growth lines. There are four costellae on the right anterior auricle, one of which bounds the byssal notch and the other three are grouped just above it. The holotype has the following dimensions: height, 13 mm.; length, 14.5; hinge length, 7 mm.; umbonal angle, 93 degrees.

Comparison—Apparently this species is distinct from others here described. Until better material is discovered, the species must be recognized by the closely spaced fila, obsolesecent byssal notch, noncostate anterior half of the left valve.

Material—The two types of E. menardi (Worthen) (Ill. St. Mus., No. 2,513) were made available to me through the help of Dr. J. Marvin Weller. The holotype, originally described as a right valve, is the reverse surface of the outer ostracum of a left valve adherent to shale matrix. An internal mold of the dorsal part of a right valve with some external ornamentation impressed upon it occurs on the same bedding surface as the holotype. The two specimens are of equivalent size and may have belonged to a single individual. Found in black, fissile, carbonaceous shale.

Occurrence—Pennsylvanian (Upper Carboniferous). Worthen notes that "the only specimen of this pretty shell in the State collection was obtained from the bituminous shale forming the roof of the Greenview coal in Menard county," Greenview, Ill. Regarding the age of the Greenview bed, Dr. Harold Wanless writes that "this is probably the roof of the No. 5 coal, which is below the upper Fort Scott limestone."

Family Amussiidae Ridewood

This family was established for the modern orbicular smooth pectinids having gills without interlamellar junctions. The type genus of the family, Amussium, has prominent auricular crura, gaping equally convex valves, and lacks a byssal notch at the adult stage, although such a notch can be seen in the growth lines at an early juvenile stage; the hinge and ligament structures are like those in the Pectinidae.

A small, compact group of pectinids having approximately the form and hinge characters of the modern Amussium is recognized from the Late Devonian or Early Carboniferous to the present time. The correspondence between the Paleozoic Pernopecten and the modern Amussium is remarkably close. The geologic history of the tribe of Pernopecten is quite different from that of the Pectinidae. The existence of a true Pecten in the Paleozoic has never been demonstrated, notwithstanding the numerous citations of the genus from the Permian. Only Pernopecten had a ligament like that of the modern Pectinidae and had acquired that kind of ligament as early as Early Carboniferous time. It scarcely seems probable that Pernopecten gave rise to the Pectinidae, however, in spite of the apparent identity of the ligament system in the two. The equivalve form, gaping valves, and obsolete byssal notch of Pernopecten suggests that it was highly specialized rather than a primitive type, and it appears probable that the Pectinidae had their origin in the Aviculopectinidae, possibly during the late Permian or Triassic.

The probable phylogeny of the Paleozoic Amussiidae is indicated in figure 17.

Genus PERNOPECTEN Winchell, 1865

Pernopecten Winchell, 1865, Philadelphia Acad. Nat. Sci., Proc., p. 125.
Entolium (part) of authors.

Smooth prosocline or acline orbicular shells, having small triangular auricles, those of the left valve extending high above the cardinal axis, whereas the dorsal margin of the right valve is straight; auricles set off sharply from the shell body, being depressed very slightly but abruptly below the general shell surface; two broad sulci extend from the beaks toward the ventral border of each valve just within the front and rear margins of the shell, producing a flattened marginal area along the front and rear parts of the shell, the posterior one being somewhat longer and producing a slight posteroventral projection of the margin; byssal notch absent in the adult, but clearly visible in the early growth lines at the base of the anterior auricle in either valve; valves with a pronounced anterior and posterior gape; shell structure of the inner ostracum concentric crossed-lamellar, that of the outer ostracum in the left valve radial crossed-lamellar, right valve prismatic, with the prisms remarkably regular hexagons set in concentric rows; auricles composed of a marginal ridge parallel to the dorsal edge in the right valve, that fits between two similar ridges on the opposite valve, operating somewhat like the so-called lateral teeth in various Pectinidae; resilifers as in Pecten or Amussium, situated below the cardinal axis.

The musculature is somewhat different from that of the Aviculopectinidae. There is no posterodorsal lobe of the adductor scars-only a very obscure expansion of the pallial line where it joins the adductor scar. This circumstance suggests that the pedal retractors were degenerate or absent, which together with the obsolete character of the byssal notch, indicates a loss of function of the foot. Scars of pedal levator muscles have not been recognized as such. The insertions at the pallial line take the form of fine radiating incisions of the shell. A scar of mantle attachment occurs below the resilifers as in Pecten or Amussium, but unlike the Pteriidae where the dorsal edge of the mantle is not attached to the shell on either valve underneath the resilifers; inner surface of cardinal margin with or without interlocking denticles.

Genotype—Aviculopecten limaformis White and Whitfield, Mississippian.

Remarks—There is a general lack of agreement as regards the genera Pernopecten; Syncyclonema, and Entolium. These are similar forms, the first being based on a Mississippian species, the second on a Cretaceous one, and the last on a Jurassic form. Hall (1885, pp. lvii-lxi) seems to have been the first to place Entolium Meek (November, 1865) in the synonymy of Pernopecten, on the supposition that the crenulations of the hinge described in Pernopecten originally existed in all of the Paleozoic forms but have been obliterated in some by wear or imperfect preservation. Girty (1909, p. 88) has followed the same course, and likewise Tryon (1884, p. 291). On the other hand, Hind (1903, p. 117) has considered both Pernopecten and Entolium as synonyms of Syncyclonema Meek, 1864. Phillippi (1900, p. 80) and Woods (1902, p. 145) likewise have considered Entolium as a synonym of Syncyclonema. Verrill (1900, p. 62) considered the three genera as distinct. Apparently none of these authorities based his synonymy on actual studies of type material and the statements in this regard are largely assumptions. These genera, as defined, have only slightly different characters and controversy has arisen over the proper evaluation of these differences.

Unfortunately, I have so far been unable to secure authentic specimens of Pernopecten limaformis and thus have been forced to rely on Hall's (1885, p. lvii) illustrations and description of the holotype and Meek's (1874, pp. 488, 489) comments on some of Winchell's specimens.

In the first place, it is obvious to anyone familiar with Paleozoic species of this group that the specimen figured by Hall as the holotype of P. limaformis is a right valve. This is significant because the genus Pernopecten was described as having a straight hinge margin. In all of the Paleozoic forms of this kind that I have seen, the dorsal margin of the right valve is straight and that of the left angulated by the extension of the auricles above the hinge. Furthermore, there is a marked gape to the valves, sufficiently obvious to permit ready detection in isolated valves if they are undistorted.

I have a specimen of Pernopecten prosseri (Mark) (=Entolium aviculatum pars of authors) and one of Pernopecten clypeatus, n. sp. before me which shows a denticulate hinge of the type that is supposed to distinguish Pernopecten from Entolium. These specimens are right valves, and associated with them are several left valves in which there is no trace of the denticulations. There are right valves of another species, clearly congeneric with P. prosseri, in which the hinge is nondenticulate. It appears to me that the absence or presence of the denticulations on either side of the resilifer is not even of specific rank, but only an individual variation. It does not seem reasonable, according to present information, to try to distinguish more than one genus of this group in the Paleozoic rocks.

I have had an opportunity to examine several well preserved specimens of Entolium demissum (Phillips) (see pl. 20, figs. 19, 20), the type of Entolium, from the brown oolitic Jura of Wiirtemburg, Germany. Superficially, there is a striking resemblance between this species and the Paleozoic forms, but it displays several features not seen in the older species: The shell structure of both valves is the same; the outer ostracum has a radial crossedlamellar structure readily visible at low magnification; there is no apparent shell gape; the auricles of the left valve are rounded instead of acuminate, and the valves are perfectly symmetrical, with a circular margin and no obliquity whatever; and finally, there is no definite byssal notch at any stage in either valve. These characters, although slight, serve to distinguish the type of Entolium from any of the Paleozoic forms that I have seen.

Measurements in Species of Pernopecten
Species	Specimen No.	Length, mm	Height, mm	Hinge length, mm	Convexity, mm	Umbonal angle, degrees
clypeatus	L1	29	29+
	L2 a	33.5	36	14.5	4	119
	L3	30	31	11.5	3.4	124
	L4	9	10	5		111
	R5	19	21	8		108
	R6	19	20	8.2		113
ohioensis	L1	20	19.5	10		115
	L2	18.5	21	12.5		103
	L3	19	21.5	10		120
	L4	21.5	22.5	10		112
	R5	16	16.5	5.6		113
	R6	16	16	7		112
	R7	19	19	7.5		115
	R8	20	18.5	6.5		119
prosseri	L1 a	31	32+	14±		113
	R1			10		113
a. Holotype. L, left valve; R, right valve.

Very little is known about the type of the Cretaceous Syncyclonema Meek (Pecten rigida Hall and Meek) except that it is a smooth shell with slightly unequal auricles and a closed margin. It is possibly a near relative of Entolium.

PERNOPECTEN PROSSERI (Mark)

Plate 20, figures 12, 13, 17, 18

?Pecten aviculatus Swallow, 1858, St. Louis Acad. Sci., Trans., vol. 1, p. 213 (not Pecten aviculatus G. von Munster, 1832).
Entolium prosseri Mark, 1912, Ohio Geol. Survey, Bull. 17, 4th ser., p. 309, pl. 15, figs. 6-8.

This is a large, sub circular, distinctly prosocline Pernopecten having high acuminate auricles on the left valve; decided gap in front and behind, much more pronounced in the left than in the right valve, as in Amussium; anterior and ventral border rather evenly rounded in the left valve, but the posterior border below the auricle slightly concave; midumbonal surface on this valve evenly convex, separated from the anterodorsal and posterodorsal margins by broad obscure sulci; auricles small with the front margin of the anterior one and the rear margin of the posterior one sigmoidally curved, i.e., the lower and middle part of the margin is broadly convex, while there is a small, abrupt concavity or sinus just below the apex of the auricle; right valve differs from left principally in having a less pronounced gap, and a straight cardinal margin; extremities of the auricles obtuse; anterior auricle almost as long as the posterior, but not as high; auricles of both valves set off abruptly from the shell body, being abruptly and slightly depressed below the general surface of the shell, producing two narrow umbonal folds which flare outward at a rapidly increasing angle; anterior auricle in each valve with a well defined byssal notch in juvenile stages, the notch becoming obsolete at maturity; inner ostracum in both valves with concentric crossed-lamellar structure; outer ostracum in left valve radial crossed-lamellar, in right valve composed of hexagonal prisms geometrically arranged in regular concentric rows; prisms having a diameter of 30 microns along a ventral line at a shell height of 26 mm.

Figure 39—Restoration of Pernopecten prosseri (Mark). Posterior view, with valves closed; the stippled parts are internal features viewed between gaping valves, X 2.8. [Image scaled and magnifications adjusted for web presentation.]

The most striking feature of the species, by which it may be distinguished from the comparable P. clypeatus, n. sp., is the lack of a shoulder or extended posterodorsal margin behind the rear auricle of the left valve. Most species of Pernopecten have a distinct emargination of the shell behind the rear auricle.

It is highly probable that this is the form that Swallow described as Pecten aviculatus, but since that species was not figured and the type specimen is destroyed, the exact nature of P. aviculatus Swallow will remain forever unknown.

Measurements of specimens of P. prosseri are given in the table previous.

Comparison—It is always difficult to distinguish between different species of Pernopecten. P. obliquus Girty and P. attenuatus (Herrick) have a notably smaller umbonal angle than the present form and are not nearly as large. P. ohioensis, n. sp., is slightly more oblique, has a differently shaped right valve, and is only about half as large as P. prosseri.

Figure 40—Anterior auricle of a right valve of Pernopecten prosseri (Mark) showing the progressive loss of the byssal notch during ontogeny; camera lucida drawing, X 8. [Image scaled and magnifications adjusted for web presentation.]

Material—Through the kindness of Prof. J. E. Carman and Dr. Grace Stewart, of Ohio State University, I have had opportunity to study the type specimens of P. proseeri. These are four syntypes (Ohio St. Univ., No. 14,036) from a single locality. From this suite I designate the specimen shown in my plate 20, fig. 17 as lectotype. Although fragmentary, these specimens show beautifully the original structures of the shell substance. Found in pure or argillaceous limestone.

Occurrence—The types came from the Ames limestone (Upper Missouri age) at New Concord, Ohio, Pennsylvanian (Upper Carboniferous.)

Figure 41—Profile of a right valve of Pernopecten prosseri (Mark) showing distribution of shell layers (P, prismatic outer ostracum; CL, crossed lamellar inner ostracum); (camera lucida drawing).

PERNOPECTEN CLYPEATUS Newell, n. sp.

Plate 1, figures 1, 14, 15; Plate 20, figures 7-11

?Pecten aviculatus Swallow, 1858, St. Louis Acad. Sci., Trans., vol. 1, p. 213 (not Pecten aviculatus G. von Munster, 1832).

Relatively large, distinctly prosocline, closely similar to Pernopecten prosseri (Mark), but especially distinguished by the development of an ernargination or crest. behind the rear auricle of the left valve, by which the shell body is extended dorsoposteriorly in a rounded convex margin behind the posterior auricle.

Measurements of specimens of P. clypeatus are given in the table previous.

Comparison—This form is especially distinguished by its robust size, and by the distinctive posterior profile.

Material—Six nearly complete specimens and several fragmentary ones from Wabaunsee beds near Thurman, Iowa (Yale Univ., Peabody Mus., Nos. 14,485, 14,486). Found in argillaceous gray limestone.

Occurrence—Upper Carboniferous. Missouri and Virgil subseries. The types came from the Howard limestone (Wabaunsee group), 2 miles north of Thurman, Iowa, and the Wakarusa limestone (Wabaunsee), south of the fault at Thurman, Iowa. There is one specimen from the Captain Creek limestone (Lansing group) near Olathe, Kan.; several specimens from an unidentified horizon in the upper Shawnee group, near Topeka, Kan.; Soldier Creek shale (W abaunsee), one half mile west of the mouth of Walnut Creek, Nebraska City, Neb.; Wakarusa limestone (W abaunsee) , Table Rock clay pit, Table Rock, Neb.; Kereford limestone (Shawnee), Snyderville, Neb.

PERNOPECTEN OHIOENSIS Newell, n. sp.

Plate 20, figures 1-3, 5, 6

Entolium aviculatum, Herrick, 1887, Denison Univ. Sci. Lab., Bull., vol. 2, p. 23, pl. 1, figs. 1, 2; pl. 3, fig. 10. ——, Morningstar, 1922, Ohio Geol. Survey, Bull. 25, p. 232, pl. 13, fig. 9.

In our collections there are two similar pernopectens, distinguished from each other by a marked difference in size and by pecularities of the right valve. The available material appears to be sufficiently complete to indicate that the larger of the two forms, P. clypeatus, n. sp., is found only in beds of Missouri and Virgil age, whereas the small form P. ohioensis, n. sp., is found only in the Des Moines subseries or equivalent rocks. The size difference may be due partly to ecological factors, because the Des Moines specimens that I have seen occur chiefiy in carboniferous limestone, whereas the postDes Moines specimens are found chiefiy in pure or argillaceous limestone, Right valves of this species are extraordinary in winglike lobes or projections at the front and rear margins. I am at loss to account for this discrepancy between right and left valves (see fig. 42) except on the assumption that the two valves did not fit snugly together along the ventral margin. There is a real stratigraphic distinction between the two forms, and this fact justifies the application of different names to them. The suite of specimens is from the lower Mercer limestone, Flint Ridge, Licking county, Ohio.

Figure 42—Diagram of Pernopecten ohioensis, X 8, right valve view, showing discrepancy in shape of the two valves; projecting margin of left valve indicated by shaded area. [Image scaled and magnifications adjusted for web presentation.]

Measurements of specimens of Pernopecten ohioensis are given in the table previous.

Comparison—As indicated in the description, the small size and peculiar form of the right valves distinguish this species. The largest Pernopecten that I have seen from beds of Des Moines age is scarcely half as large as a full-grown P. prosseri or P. clypeatus.

Material—The species is based on 21 specimens, right and left valves, from a single horizon and general locality. The holotype and several topoparatypes are U. S. Nat. Mus., No. 89,789. There are several topoparatypes at the University of Kansas, and two at Ohio State University, No. 4,604. In these specimens the inner ostracum has not been preserved, but the outer ostracum in both valves shows the original structure. There are in addition several other specimens from various localities. Found in black and dark-gray carbonaceous limestone.

Occurrence—Upper Carboniferous. The holotype and topoparatypes came from the lower Mercer limestone, Flint Ridge, Licking county, Ohio; other paratypes are from the Liverpool formation, SW sec. 15, Knox Township, Knox county, Illinois; "Coal Measures" (probably Cherokee) Henry county, Missouri; Boggy shale, NW sec. 27, T. 3" N., R. 7 E., Okla.

PERNOPECTEN ATTENUATUS (Herrick)

Plate 20, figure 4

Entolium attenuatum Herrick, 1887, Denison Univ. Sci. Lab., Bull., vol. 2, p. 24, pl. 1, fig. 11.

This rare species is comparable in characters of the left valve to P. ohioensis with which it is associated, and differs chiefly in its considerably greater relative height and in its smaller umbonal angle.

I have seen only one example that might belong to the species. In my specimen the umbonal angle is approximately 106 degrees. The height is 28 mm.; and the length 24 mm. Morningstar (1922, p. 232) gives the height of a topotype specimen as 18 mm., and the length as 13 mm.

P. attenuatus seems also to be distinguished from most species by the absence of a posterodorsal shoulder, in this respect being comparable only with P. prosseri.

Comparison—This form is especially distinguished by its relatively high, narrow proportions.

Material—I have seen only one specimen of this species. Found in black, pyritiferous limestone; dark-gray, silty, hard shale.

Occurrence—Upper Carboniferous. My specimen (Yale Peabody Mus., No. 14,493) came from the roof of the Danville coal at Danville, Ill. (Des Moines subseries). The roof of the Danville coal is probably just below the horizon of the Pawnee limestone, according to Dr. Harold Wanless (personal communication). The original types, which appear to be lost, came from the lower Mercer limestone (Pottsville), Flint Ridge, Ohio.

PERNOPECTEN OBLIQUUS Girty

Plate 20, figures 15, 16

Pernipecten? obliquus Girty, 1908, U. S. Geol. Survey, Prof. Paper 58, pl. 9, figs. 13-14a.

Girty's description:

Shell small, elongate, nearly flat. Hinge line shorter than the width below. In the left valve the two ears are nearly equal, small, and moderately well defined, both in outline and by the grooves on the surface. The shell expands considerably below them, and is gently inclined backward. The superior portion is more or less turned upward at the sides along two converging lines.

I have had the privilege of studying Doctor Girty's type specimen, and from microscopic examination of the specimen under xylol it appears to me that the shape indicated by Girty's fig. 13 is incorrect. The auricles extend high above the hinge, as is normal in the genus.

The species appears to be a small one, comparable in size to P. ohioensis, and would be difficult to identify in the absence of good material. However, it appears to have a rather distinctive posterior profile, being somewhat more produced posteriorly than P. ohioensis and, unlike that species, lacks a well-defined posterodorsal shoulder or lobation of the margin below the rear auricle. Right valve unknown.

Material—I have seen only the holotype of the species and one topoparatype, both left valves, the two figured by Girty. They bear the catalogue numbers 405, 406. Found in olomitic limestone.

Occurrence—Upper Permian, Capitan limestone, Guadalupe Mountains, western Texas.

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