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 fifths 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. 



(162.) 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 f, &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. 119. f. 1124, 1125. f z o1 - Illustrations, 1st Series. 

 J Exotic Conch. 



