CHAP. VII. TROCHIDyE. PRIMARY DIVISIONS. 21 



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mary divisions^ and hold the rank of sub-families : 

 — 1. The Phasianellin^, Lam., where the shell is 

 spiral and obovate, and shaped like a Butimus ; the 

 outside is polished, and the operculum shelly. 2. The 

 Senectin^*, or sea snails, resembling the garden snail 

 in form, but perlaceous, and furnished with a thick, 

 round, shelly operculum. 3. The Trochin^, or tro- 

 chuses, having the shape more pyramidical, the body- 

 whorl flattened, and the aperture closed by a horny 

 operculum. 4. The Rotellin^, or wheel-shells, 

 which are also perlaceous, and nearly discoid in shape, 

 with a thickened mass over the inner lip. 5. Pleuro- 

 TOMARiA Tief., — a fossil trochiform shell, having a 

 slit on the outer lip, as in the genus Pleurotoma 

 of Lamarck. Such are the primary forms, which 

 seem to belong to the TROcniDiE. Our information 

 on the animals is partial ; but there is enough to 

 guide us in three of the chief groups. Thus the ques- 

 tion whether Phasianella belongs to this family or the 

 TurhidcB has been set at rest by M. Quoy, among 

 whose beautiful figures is the animal of the typical 

 species : the same eminent zoologist has also decided 

 the relations of Senectus to the TrochincB, by figuring 

 the Turbo sarmaticus, — thus showing its affinity to 

 the animal of Trochus. The gradual chain of con- 

 nection between Trochus, Solarium, and Rotella, leaves 

 us in no doubt that these also form part of the family ; 

 but whether Pleurotomaria is merely a genus of the 

 latter group, or the representative of a sub-family, 

 must still remain a disputed point. We insert it, how- 

 ever, under the latter supposition, because it will appear 

 by the following analysis, that it can in no wise be in- 

 corporated elsewhere. 



(200.) The Phasianellinj=;, or pheasant-snails, 

 form one of the most isolated genera in the whole of 

 the Testacea. That they represent the TurbidcE, is 

 obvious ; for Lamarck and his followers have mixed 



* Senectus of Humphrey, Marmarostoyna (pars) Sw., Turba of Cu- 

 vier, &c. 



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