CHAP. VI. THE LUCERNELL^. IQS 



from which it is nevertheless separated hy its circular 

 aperture, analogous in this group, to Cyclo.stonia. As 

 this is the European type, we have preferred retaining to 

 it the sub-generic name of Caracolla, that the nomen- 

 clature of such well-known species should not be dis- 

 turbed. The whole group is composed of granulated 

 shells. 



(17.9-) 11"^ the second, or sub- typical group, we shall 

 find five sub-genera agreeing with those of the last. As 

 a general character, the Lucernellcp may be known by the 

 inner lip, no less than the outer, being provided with 

 teeth : those on the former are either one or (very rarely) 

 two, placed almost transversely, as if to guard the en- 

 trance to the shell by rendering the aperture remarkably 

 narrow ; the teeth on the outer lip are situated at its in- 

 ner edge, and have no corresponding indented grooves (as 

 in Lucerna) on the outer surface. All the LucernellcB, 

 likewise, are convex on their sides, not carinated. It is 

 only in the sub-genera Hemicychi and Lucidella that the 

 inner teeth disappear ; but the union of the first of these 

 two with Cyclodoma is so obvious, that they cannot be 

 placed in different genera. The first form on quitting 

 Anastoma is Polydontes, of which the type is that sin- 

 gular shell P. imperaior oi Montfort; it is remarkable for 

 its aperture being surrounded by nodulous teeth, obtuse, 

 and resembling large granules, while those of Anastoma 

 are more properly folds or plaits : following this comes 

 Lucernelln , where the form of the shell is often globose, 

 the umbilicus closed, and the teeth very complicated : these 

 lead to the greatly depressed form of Cyclodoma, where 

 the shape is sometimes as discoid as in the well-known 

 genus Planorhis. These shells are almost always striated; 

 and, although small, are highly interesting. A great 

 number of species inhabit the mountains of North Ame- 

 rica and Madeira, but probably not one half of those 

 that exist have yet teen discovered. In HemicycJa, the 

 internal or left-hand tooth either entirely disappears, or 

 is reduced to a little tubercle ; the outer lip makes a bold 

 and dUated semicircle^ the margin of which is broad and 

 o 



