CHAP. VI. THE LIMACIN^, OR NAKED SLUGS. 189 



taken for freshwater limpets ; so that the analogy is 

 most complete. Potomophila, again^ is the only type 

 having any fold upon the pillar ; and Clausilia is the 

 only genus so distinguished among the Achatince. It is 

 the excessive enlargement of the body-whorl which 

 gives such a peculiar character to the turbinated snails ; 

 and this renders them distinct from the Achatince : and 

 the very same disproportion is observed in all the 

 typical examples of Limncpus, of which the common 

 L. stagnalis is a familiar example. The near approxi- 

 mation of the amphibious genus Succinia to the group 

 we have now gone through^ is abundantly obvious. 

 While, from possessing four tentaeula, with the eyes 

 pedunculated, it cannot be brought within the limits of 

 the freshwater shells ; it is, in short, as Cuvier has 

 happily expressed it, ''^ a Testacella, with a very large 

 shell." As the systematic definitions will be here- 

 after given, we need not dwell longer upon this sub- 

 family. 



(174.) The sub-family of Limacina; which contain 

 the naked slugs, is admitted to be such a natural group^ 

 that this part of our survey may be considerably abridged. 

 The first of the typical genera appears to be Limcijc, 

 where the tentacula are four, and, in general, the 

 vestige of a shell is placed near the extremity of the 

 body. The second is typically represented by the 

 genus Herpa of Guilding, in which the tentacula are 

 either entirely wanting, or, as in the sub-genus On- 

 chidium, they are only two in number, and very short. 

 The determination of Herpa, as connected to Li max, 

 is one of the most important discoveries in mala- 

 cology that has been made for many years, not merely 

 as to the fact itself, but to the inferences to which it 

 leads : it may be considered, in short, as the germ, 

 or first incipient developement of this immense family, 

 from which all its innumerable modifications branch 

 off. Its analogy to Planaria among the Pareiichymata 

 is abundantly obvious ; a relation well understood by 

 its learned and acute discoverer, who thoroughly un- 



