170 



SHELLS AND SHELL-FISH. 



PART I. 



ually prove the real types of this group^ which repre- 

 sents Cyclostoma among the Helicin-s;. 



(159.) The AcHATiN^, as we have already shown, 

 constitute the sub-typical group, representing in their 

 own family the zoophagous tribe. This analogy is seen 

 in two ways : first, they are the most spiral of all the 

 Helicidce, just as the Zoophaga are the most spiral of 

 all the Testacea ; secondly, they are the only ones where 

 the indication of a basal channel is seen, as if to pre- 

 pare us for the full development of that character in the 

 next great tribe which succeeds this. But setting aside 

 all these considerations, at least for the present, we will 

 at once proceed to characterise the five genera into 

 which they appear to divide themselves. 1. In Achatina, 

 properly so called (^A.marginata^^w.Jig. 23.), the basal 



extremity of the pillar is trun- 

 cated, so as to present a distinct 

 separation between it and the 

 outer lip ; this lip also is always 

 thin and sharp-edged at every 

 period of age. 2. In BuUmus, 

 on the contrary, the outer lip 

 is more or less thickened and 

 reflected backward ; while the 

 inner lip — always absent in 

 Achatina — forms either a little 

 fold just below or near to the 

 umbilicus, or is continued up, 

 and spreads a polish over the 

 left side of the aperture : the general shape of the shell 

 in the typical sub-genera of both this and the last is 

 obovate, having the body-whorl much the largest, and the 

 aperture without teeth, except in the sub-genus Auri- 

 cula. This leads to the next or 8d genus, Clausilia : in 

 these the spire is excessively long in proportion to the 

 aperture, the basal or body-whorl being hardly larger than 

 the others ; and the aperture, moreover, is furnished 

 with various folds, or toothlike process. The ^th, 

 Helicina {Jig. 18. b), is distinguished at first sight by its 



