GASTEROPODS. 185 



specimen The illustration, however, represents the gastero- 

 pod attached to the side of .the common and well-known erin- 

 oid Platycrinus hemisphericus Meek and Worthen, so that 

 not the slightest doubt is entertained as to which shell it was 

 intended to portray. Besides, at Orawfordsville, this inollusk is 

 almost invariably associated with the crinoid just mentioned. 

 Only two other species of the genus C. equilateralis and <?. 

 sulcatinus are known to occur at the Indiana locality, and 

 these are both very different. Taking all things into consider- 

 ation, therefore, and remembering that Hall's name Platyceras 

 subrectum had been preoccupied, it seems advisable to restore 

 Owen's title for the shell in question. 



The form most closely allied to this species is C. fisurella 

 (Hall), from which it is distinguished in being proportionally 

 more elongate, while the apical part of the shell is character- 

 istically slender. Ordinarily the shell is more or less conspciu- 

 ously plicate, but the folds are, for the most part, narrow, and 

 usually irregular and broken. 



For an elongated specimen, Meek & Worthen have indi- 

 cated the name Platyceras extinctor "should it prove distinct," 

 but the term cannot be regarded as having been actually pro- 

 posed, while the form itself is manifestly only an attenuated 

 internal cast of 1. pabulocrinus. It, however, exhibits well 

 the characteristic muscular impressions. 



This species, like O- equilateralis (Hall), with which it is 

 usually associated, occurs in the Burlington limestone and 

 ranges through the Keokuk. Its association with crinoids at 

 Crawfordsville, Indiana, has been for the most part with Platy- 

 crinus he mi spheric us M. & W. ; while at Burliugton it ad- 

 heres to a structurally similar form, Eucladocrinus millebra- 

 chiatus W. & Spr. The vault in the first species is very much 

 elevated, and the anal opening is situated laterally between, 

 and slightly above, two arm bases. The dorsal cup is orna- 

 mented by numerous conspicuous rounded tubercles. As the 

 shell increased in size the pliant apertural margin encountered 

 successively the different nodes, which caused the lip at these 

 points to deflect outward, giving rise to variously shaped pro- 



