GASTEROPODS. 193 



stoma; and that from these it was the intention of Conrad to 

 construct a correct generic term. In this attempt he used by 

 mistake the connecting o, which is necessary in forming com- 

 pounds with many Greek words, but which in the present 

 instance was not called for, since the term is formed directly 

 without the intervention of a copulatory vowel. For these 

 reasons Conrad's and Klein's proposals cannot be regarded as 

 distinct. 



Since it is clear, then, that Platystoma cannot be retained 

 for the American group of Paleozoic gasteropods, some other 

 term must be adopted. Fisher has taken advantage of the 

 questionable validity of Conrad's name, and has recently pro- 

 posed Diaphorostoma for the same group, with Platystoma 

 niagarensis Hall for his type. This writer places Hall's Stro- 

 phostylus as a subgenus under Diaphorostoma ; while Zittel 

 assigns Hall's section to a similar taxonomic rank, but under 

 Platystoma. Had Conrad's type of Platystoma not been a form 

 midway between the two extremes of a series forming the 

 group, Fisher's name might be retained ; but the type of Stro- 

 phostylus is congeneric with P. ventricosum ; and as this name 

 was the next proposed, it must be taken into consideration. 

 Were it not for this fact, two sections of this group of shells 

 could be with great propriety made. As regards the term 

 Strophostylus, it was established by Hall in 1859 for certain 

 shells closely related to Platystoma, but differing chiefly in 

 having a slightly " twisted or spirally grooved columella." 



Recently a large series of the most important species of 

 both Platystoma and Strophostylus was examined and the ma- 

 trix carefully removed from the apertural portions of many of 

 the shells. The structural features disclosed in the various 

 forms show a relationship between the two established genera 

 that was long suspected. It is well known that the type of Con- 

 rad's genus Platystoma ventricosum Conrad is a somewhat 

 globose shell with a small spire elevated slightly, and having 

 the aperture broadly ovate. The inner lip is somewhat thick- 

 ened and subangular within, giving a slight indication of a col- 

 G 14 



