GASTEROPODS. 179 



is relatively very small. The longitudinal folds are not unfre- 

 quently very pronounced, and being few in number, impart a 

 peculiar trilobate appearance to the shells ; in other examples 

 all traces of plications are wanting. 



The extensive series of C. equilateralis from the Craw- 

 fordsville shales has been very thoroughly studied of late, dis- 

 closing many interesting phases of the habits of these gastero- 

 pods, hitherto not elsewhere presented in such an eminently 

 satisfactory manner. At this locality the shell is usually at- 

 tached to the calyx of Gilbertsocrinus tuberosm (Lyon and 

 Casseday ), but the mollusk is not invariably associated with 

 this particular species of crinoid, as Meek and Worthen sup- 

 posed. A number of typical examples of the Oapulus in ques- 

 tion have been observed adhering to Platycrinus hemispheri- 

 cus ( Meek and Worthen), with which, however, is more com- 

 monly associated I. pabulocrinus (Owen). In Gilbertsocrinus 

 the vault is relatively large, nearly flat, with the anal opening 

 located midway between the center and margin. In both G. 

 tuberosus (L. & 0.), from the Keokuk shales, and G. typus 

 (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, necessa- 

 rily 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 conspiculously evi- 

 dent in the shells of many ancient Gapuli, is thus capable of 

 being traced, and especially in those examples in which the no- 

 dosity of the dome plates of the crinoid has reached a high 

 development. This phenomenon of nonparallelism of the lines 

 of growth is not therefore indicative of a change in station of 

 the gasteropod, as has been suggested at various times. 



It has been clearly shown elsewhere that shells of C. equi- 

 lateralis, when adhering to flat surfaces, are always very much 

 depressed and have the aperture proportionately much more 



