70 The Dentition of British Mollusca. 



three areas are found, the outer ones are to be considered as 

 the pleurce, inasmuch as there is not unfrequently a manifest 

 division in the membrane between them and the lateral areas ; 

 but never, as far as I have observed, between the latter and the 

 median region. This arrangement is typical of a large class, 

 having the formula 3.1.3, which embraces genera so dissimilar as 

 Cypro3a,Aporrhais,an&Natica, together with the vegetable feed- 

 ing Littorinidce, and the operculated land and fresh water mol- 

 lusks. Again, when only two areas exist, it seems probable that 

 this is caused by the absence of the central ones, and the teeth 

 should therefore be termed uncini. Such is the case with the 

 Bullidce aud the allied bare-gilled family of the Doridce; and 

 this conjecture is confirmed by the fact, that in Cylichna and 

 its nearest allies, which are transition genera, a minute central 

 tooth is present. 



This subject has been investigated by several naturalists ; 

 abroad, by Loven and Troschel, and at home by Gray and 

 Woodward, with a view to obtaining criteria for a syste- 

 matic arrangement of Gastei-opodous Mollusca. Up to the 

 present time, however, their labours have only partially suc- 

 ceeded. The union under one formula of so many creatures 

 widely differing in shell, anatomy, and habits, clearly indicates, 

 that if the lingual ribbon contains generic characters, they have 

 not yet been ascertained. At the same time, it does present 

 differences which may offer collateral evidence in cases difficult 

 of discrimination. It does not help us to separate carnivorous 

 from phytophagous animals ; but it seems possible to make use 

 of it as a mark between species. For, in all the examples I 

 have examined, there is a distinct difference between the tongues 

 even of the most closely allied. Chiton discrcpans is hard 

 to tell from C. fasciculans by the outer parts alone ; but the 

 tongues are clearly distinct. Patella athletica may, it is said/ 

 be similarly divided from P. vidgata. The two British species 

 of Acmcea afford remarkable differences. Trochus ziziphinus 

 and the nearly allied T. granulatus is another case in point. 

 On the other hand, the occurrence in T. hclicinus of six laterals 

 is one of the reasons Avhich suggest a change in its generic 

 name ; and great lingual dissimilarity demands the separation 

 of our two fresh-water Ancyli. In this way supposed varieties 

 may be possibly decided. If, for instance, the lingual ribbon 

 of the many subdivisions of Litorina rudis is constant in its 

 characters, they cannot bo received as species. Again, the 

 position of the fluviatile Paludinidcb in close proximity to 

 the sea-loving Litorinidoe is confirmed by the likeness of 

 their dentition; while Neritvna jluviatilis, with the formula 

 co. 3. 1. 3. co, shows an approach to the genus Trochus. 

 All the land and fresh-water mollusks without opercula 



