172 GASTEROPODS. 



most of them are merely internal casts. They form, however, 

 Tan important feature of the fauna inclosed in these rocks. The 

 Burlington and Keokuk species are very closely related, and in 

 part extend through both epochs, after which the genus is of 

 rare occurrence in the continental interior. It is of consider- 

 able interest to note that this numerical reduction after the 

 close of the Keokuk was accompanied by a marked depauper- 

 ization of the individuals which struggled through to the end 

 of the Paleozoic. Through all the St. Louis, Kaskaskia and 

 Coal Measures the species, without exception, are diminutive. 

 The 0. acutirostre of the St. Louis became reduced to nearly 

 one-half the size it possessed in the Keokuk, notwithstanding 

 the fact that this species had perhaps a wider geographical 

 range than any other congeneric form occurring within the 

 Mississippi basin, and was therefore better adapted to preserve 

 its full vigor, at least in some parts of its distribution. 



Structural Characters. Shell depressed, subglobose, ob- 

 liquely subconic; body-whorl very large. Aperture ample, 

 expanded ; labrum more or less sinuous, inner lip not anchy- 

 losed to the spire. Surface glabrate, plicate, or sometimes 

 spiniferons; lines of growth often urabricate. 



The shells which have been referred to Platyceras pre- 

 sent a manifold variety of forms. It is, therefore, not improb 

 able that a fuller examination and comparison of all the known 

 species will demand a somewhat different arrangement and 

 subdivision of the group than that now existing. In this sec- 

 tion the shell presents few salient characters for consideration. 

 As already stated, it is often with extreme difficulty that the 

 forms of this group can be satisfactoiily separated from cer- 

 tain varieties of Platystoma and various genera of Patelloid 

 shells. In general, however, the test of Capulus is coiled, 

 subspiral, arcuate or subconic, with a relatively small spire and 

 an immense, rapidly expanding body whorl, while the surface 

 is usually without ornamentation. The large majority of the 

 species of this group possess tough, massive shells, which 

 are generally, therefore, in a much better state of preservation 

 than most of the associated molluscan remains. 



