172 PROCEEDINGS OF THE ACADEMY OF [1890. 



This species seems to be one of the most ahundaut gasteropods of 

 the Keokuk beds ; especially at Crawfordsville, Indiana, where the 

 conditions of environment, during the deposition of the blue clayey 

 shales of that locality, wei'e uncommonly favorable to the develop- 

 ment of this mollusk. Some of the shells from the locality mentioned 

 have very considerable measurements : height 45 mm. ; length along 

 the dorsum, 95 mm.; breadth, 60 mm. Not only is the species 

 under consideration variable in size but it is extremely so in form 

 and in the configuration of the apertural margin. Perhaps no 

 Capulus in all the Carbonic presents so wide a range of variation as 

 does this species. Immature shells appear to be glabrate, but as 

 growth proceeded they became more and more rugose and iml^ricate. 

 The spire is as often contiguous as free and simply incurved ; and in 

 adult specimens it is relatively very small. The longitudinal folds 

 are not unfrequently very pronounced and being few in number, im- 

 part a peculiar trilobate appearance to the shells ; in other examjiles 

 all traces of plications are wanting. 



The extensive series of C. equilateralis from the Crawfordsville 

 shales affords many interesting phases of the habits of these gaster- 

 opods, hitherto not elsewhere presented in such an eminently satis- 

 factory manner. At this locality C. equilateralis is usually attached 

 to the calyx of Gilbertsocrinus tuberosus (Lyon & Casseday) but the 

 mollusk is not invariably associated with this particular species of 

 crinoid, as Meek & Worthen^ supposed. A number of typical ex- 

 amples of the Capulus in question have l)een observed adhering to 

 Platijcrinus hemisphericus ]Meek & AVorthen [plate II, fig. 11] with 

 which, however, is more commonly associated C. infimdibulum (M. 

 & W.). In Gilbertsocrinus the vault is relatively large, nearly flat, 

 with the anal opening located midway lietween the center and mar- 

 gin. In both G. tuberosus (L. & C.) from the Keokuk shales and 

 G. tyjnis (Hall) from the Burlington limestone, the ventral plates 

 are convex, or, as in many specimens very nodose. The growing 

 margin of the gasteropod shell having adapted itself exactly to the 

 irregularities of the surface of the crinoidal vault, necessarily was 

 always more or less deeply sinuous, each sinus being produced by the 

 nodosity of the vault plate in contact ; while the small linguiform 

 projection between two sinuses extended down between the nodes of 

 two contiguous plates. The extreme nonparallelism of the lines of 

 growth so conspicuously evident in the shells of many ancient 



^ Geol. Sur. Illinois, vol. V, p. 335. 



