192 GASTEROPODS. 



and the configuration of the apertural margin may not be en- 

 tirely dependent upon the immediate surface in contact; but 

 from a long-continued habit of adhering to a crinoid presenting 

 such remarkable ventral features as Pterotocrinus, the gaster- 

 opod gradually acquired, after many generations, a decided 

 tendency toward the quinquelobate form, which made itself 

 manifest at an early period of the mollusk's existence, and per- 

 haps even in the latter part of the embryonic stage. 



In order to bring the mouth over the ventral aperture of 

 the crinoid, and at the same time rest securely on the limited, 

 flattened, summit, at one side of which the anal opening was sit- 

 uated it was necessary for the gasteropod to have the anterior 

 portion of the shell directed toward the posterior side of the 

 crinoid one of the few instances of the kind that have been 

 noted ; for almost invariably the front of the gasteropod shell 

 is directed toward the anterior side of the echinoderm. 



Genus Strophostylus HALL. 



The group embraces a considerable number of familiar 

 species, ranging in geologic time from the Niagara epoch to 

 the close of the Paleozoic. It is rather unfortunate, therefore, 

 that Conrad's term, Platystoma, had been preoccupied, having 

 been used in generic diagnoses on at least four different 

 occasions. Megerle early applied this name to certain mol- 

 lusks closely related to Baccinum ; but so far as is known, no 

 formal publication of the term was ever made. Were this the 

 only obstacle in the way, Conrad's genus might be allowed to 

 stand, for the reason that Megerle's proposition was only in 

 manuscript. Klein, however, proposed Platystoma in 1753 for 

 a genns of Cyclostomacea. Meigen adopted the same term in 

 1803 for certain flies, and Agassiz, in 1829, also used it for a 

 section of Silurid fishes. The preoccupation of Conrad's 

 Platyostoma by Klein's Platystoma, like a number of similar 

 cases, has been objected to on the ground that the two terms, 

 while derived from the same words, are not identical, because 

 Conrad's compound has a connecting o. It is quite manifest, 

 however, that both generic words are taken from platys and 



