Original published in D.F. Merriam, ed., 1964, Symposium on cyclic sedimentation: Kansas Geological Survey, Bulletin 169, pp. 541-563

Figure 10--Concretions from Protocardia-Oxytoma and Idonearca Assemblage Zones. A, Fragment with Protocardia. B, Fragment with Oxytoma. C, Part of a Idonearca concretion.