194 GASTEROPODS. 



umella. In all respects it very closely resembles the typical 

 forms of Strophostylus cyclostomus Hall, from the Niagara 

 shales of Waldron, Indiana. The shells described under Platy- 

 stoma subsequent to the appearance of the original diagnosis 

 vary more or less from the typical species. The general tend- 

 ency, however, has been to include under Conrad's genus those 

 forms having a rather small, depressed spire, the inner lip rather 

 thin, and simply reflected over the body-whorl. Often the lip 

 does not touch the second volution, and the mouth of the shell 

 is frequently free for a considerable distance. Some of these 

 forms closely approach Paleozoic species of Capulus. 



When, in 18,59, Hall examined the interior of a Platystoma- 

 like shell (which he afterward called Strophostylus andrewsi\ 

 he found that the specimen had the inner lip considerably 

 thickened and somewhat angular within. As a matter of 

 course, the interior angularity appeared twisted on account of 

 the continued enlargement of the shell to the adult stage. In 

 some individuals the thickening was considerably exaggerated, 

 and formed a short projecting lamella or ridge. But from the 

 figures given by Hall it is clear that this was not entirely char- 

 acteristic, and the two specimens figured in the text differ 

 widely in this particular. Nevertheless, Strophostylus was 

 proposed, and has since been applied to nearly a score of 

 species. 



The limits of Conrad's and Hall's genera have never been 

 precisely defined in the descriptions of species, and the larger 

 majority of the forms in question have manifestly been as- 

 signed to their respective groups, more on account of the gen- 

 eral shape of the shell than from an examination of the 

 columella parts, which were only in exceptional cases visible 

 without the removal of the hard matrix. 



From the foregoing it is evident that Strophostylus and 

 Platystoma are practically identical, and that, therefore, the 

 two must be regarded as synonymous. The type of the first 

 section, Strophostylus andrewsi, actually stands at one ex- 

 tremity of a rather extensive and variant series of shells, of 

 which Platystoma ventricosum is one of the median members. 



