CHAP. VI. 



SUB-GENERA OF HELIX, ETC. 



165 



are never any on the outer lip. In the second, or Helia; 

 proper, the spire is more elevated, the shell more globose, 

 the umbilicus covered, and the outer lip more or less 

 reflected : these obviously lead to Zonites, which chiefly 

 differs in the outer lip being always thin, the spire more 

 depressed, and the umbilicus more or less open. By 

 degrees, however, the spire becomes more 

 and more elevated, until, in that sin- 

 gular shell Epistylia conica* {fig. 18. «), 

 we have a Papa and a Helix combined in 

 one form. The last is a very beautiful 

 type, already characterised as a sub- 

 genus by Mr. Gray, under the appro- 

 priate name of Strepsaxis, while its 

 obvious affinity to Hemicycla brings us 

 again to the point whence we commenced. 

 (156.) The genus Geotrochus, in like manner, 

 seems to take the following course, corresponding to 

 that of Helix. We give the name of 

 Hemitrochus to certain trochiform shells, 

 {fig. \^.),hai\mg the spire nearly as elevated 

 as in Epi.^thjlia, but less thick ; the whorls 

 much fewer, and the inner lip margined 

 internally ; the basal whorl is not angu- 

 lated, so that, if the spire were not conic, and the inner 

 lip not thickened within, they might be arranged with 

 Zonites. These shells at once bring us to the typical 

 sub-genus Geotrochus ^roYter, where the body- whorl in its 

 widest circumference is more or less carinated, and the 

 spire gradually tapering to a pointed cone ; it is, in fact, 

 just the shape seen in the marine Trochuses : in some 

 few species, chiefly small and European, the margin 

 of the lip is thin, but in the more typical ones from 

 tropical America it is thickened, dilated, and almost re- 

 flected. Our next sub-genus, Pithohelix is as spiral as the 

 last ; the spire, however, is obtuse at the top, the body 

 whorl not angulated, and the aperture, consequently, 

 rather higher than broad : such as have the spire 



* Helix episiylium of authors. 

 M 3 



