198 



SHELLS AND SHELL-FISH. 



PART I. 



we also place the European 

 P. impura Lam.^ and the 

 West Indian P. parmda of 

 Guilding (^^.36.), the ani- 

 mal of which, according to 

 his drawings, has the tenta- 

 cula unequal, — that nearest 

 the pillar being almost twice 

 the length of the other. It 

 is as a sub-genus, also, of 

 Paludina, that we are dis- 

 posed to regard Valvata. 

 The shells of this last remarkable type are mostly of the 

 same form as many of the helix-like Cyclostomce ; the 

 aperture is also round, and closed with an operculum. 

 The animal, which we have not seen, is described by 

 Midler as having the branchia, or gills, pectinated, and 

 projecting from under the mantle, floating externally, 

 and vibrating every time the animal breathes : on the 

 right side of the body is a filament, which resembles 

 a third tentaculum. 



(183.) Our third sub-family, Melanian^, is com- 

 posed of those long-spired fluviatile shells which form 

 the genera il/e/anm and Melanopsis, together with that of 

 Planascis, as the most aberrant, and two others now first 

 designated as Paludomus and Ccrithidea. This, which 

 we think is the typical sub-family of the Turbidce, is 

 so numerous, that it becomes necessary to characterise 

 the sub-genera ; for without this, the theory of their re- 

 presentation could not be rendered intelligible. The 

 animals of these shells are well distinguished from the 

 last, by having their eyes more developed, and placed in 

 the middle of the tentacula, while the mouth is elongated 

 in the form of a proboscis. The genus which makes the 

 nearest approach to the AmpnUarino', in the globular 

 form of the shell, the short spire, and the rotundity of 

 the aperture, is that of Paludomus Sw., formed to re- 

 ceive those short-spired shells which at present are placed 

 in that of Melania. The American species form the sub- 



