174 SHELLS AND SHELL-FISH. PART I. 



and the other begins. Leucostoma, however, is an 

 exception ; it is more isolated, and consequently its pre- 

 sent position is very doubtful. There is also a hiatus 

 between Leucostoma and Achatinella; but then this last 

 is so perfectly connected to Achatina, that nothing can 

 separate them. Thus we have four lifths of a circle ab- 

 solutely perfect; so that, if Leucostoma had been undis- 

 covered, and the fifth type entirely wanting, still this 

 deficiency would not affect the general principles of our 

 arrangement, — we should still have four of the sub- 

 genera intimately representing four of the genera ; and 

 we should conclude, by inductive reasoning, that the 

 fifth of the former, when discovered, would represent 

 the fifth of the latter. 



(l6i}.) The most numerous in species, and hence 

 the most varied in their forms, of all these sub-genera, 

 is that of Achatina proper. In consequence of this va- 

 riation, the species, as is usual, assume the forms of 

 the genera and families which surround them ; so that 

 they might be called sectional types. An instance of this 

 we have already shown in Achatina virginea, which, with 

 its allies, obviously represents Achatinella ; then comes 

 Achatina Zebra,, &c. as the pre-eminent types of all : 

 following this, we have such shells as A. elongata Sw.* 

 &c., with their longer spire, and thus assuming one of 

 the chief characters of Cochlicopa ; while A. crenata t, 

 and perversa j, &c., by their still more produced 

 spire, immediately remind us of Macrospira : the de- 

 pressed form of Ach. Sultana ^ finds its prototype in the 

 unusually depressed Helicince ; and thus we have, in 

 this single sub-genus, indications of all the genera of 

 the sub-family. This is sufficient to show the neces- 

 sity of rigid analysis, before we can determine what is 

 the rank of a particular division, whether it be sec- 

 tional, sub-generic, or even generic : the same may be 

 said of higher assemblages ; for it is impossible to know, 

 a priori, what groups are of the rank of sub-families or 



* Chem. pi. 11!». f 1124, 1125. f Zool. Illustrations, 1st Series. 

 J Exotic Conch. 



