CHAP. VL. THE HELICIN-ffi. l63 



we have stated them merely as a preliminary step, to 

 show the general, not the particular,, resemblance of 

 the primary groups. The extreme difficulty attending 

 such an analysis of the HeUcince and the Lucernince, as 

 is necessary to determine the sub-generic types, will 

 prevent us, for the present, from entering into the de- 

 tails of these two groups so fully as we shall do in regard 

 to the others. We commence with the HELICIN^E, as 

 the most typical group ; but, from the above cause, its 

 analysis will be less minute than that of the other 

 divisions. 



(154.) The typical sub-family of HELICIN^, although 

 more numerous, perhaps, than any of the land -shells, 

 has received less attention, in regard to its natural di- 

 visions, than any group of the same rank. This has 

 probably originated in two causes ; the entire similitude 

 that runs through the animals, and the little variation, 

 of a decided and very prominent nature, observable in 

 the shells. Nevertheless, a closer attention leads to the 

 belief that this group, within itself, will be found as 

 diversified in its modifications as any, less obtrusive, 

 indeed, but promising the same results, when thoroughly 

 analysed, as in all natural groups. As a whole they 

 are immediately known from the Lucernina, or land- 

 volutes, by never exhibiting a discoid shell furnished 

 with plaits or teeth : the aperture of one division, indeed 

 (Pupa), has these appendages ; but then the form of 

 all the shells is spiral and elongated, and cannot there- 

 fore be confounded with the Lucernince. The Helicidce, 

 perhaps, are best distinguished by the aperture being 

 without teeth and transversely oval, a shape which re- 

 sults from the depressed form of the body-whorl, which^ 

 in the Achatince, is always oblong ; and this character is 

 the more important, because it is preserved even in such 

 genera as Geotrochus, whose spire is often as conical and as 

 much produced as that of the Bulimi. It is also to be ob- 

 served that, with very few exceptions, these half-spiral 

 snails have the tips of the spires very obtuse arid even flat- 

 tened; whereas, in the AchatirMZ, they are always pointed 

 M 2 



