MOLLUSC A. 83 



and others the fresh waters. The mantle is simple, without fringe or siphon ; 

 the head ends in a short trunk, and the food is vegetable, chiefly decaying 

 algfe. The family contains a number of sub-families, the limits of which 

 are not well ascertained. Melania amarula i^pl. 75, fig. 90). 



Littorina is a genus of small marine shells which inhabit the coasts. 

 Z. tenebrosa^ which inhabits both sides of the North Atlantic, leaves the 

 water for hours, climbing up the grass of the salt meadows. According to 

 Dr. Gould, it may be found " at a considerable distance from any water." 

 He remarks, also, that "it lives a week or more after being removed from 

 the water." The species vary very much in their specific characters. 

 Planaxis and Eulima are placed in this family, as well as Turritella (a shell 

 much like j!?Z. T^^fig. 117, but with the aperture entire), but as the mantle 

 is stated to be scolloped, it seems to be more nearly allied to the Cerithiidas. 

 The mantle of Scalar'm {j^l. 75, fig. 101, S. clathrus ; fig. 102, S. scalaris) 

 not having been described, its place remains doubtful, although it probably 

 comes near to Turritella. 



Paludina is a genus of freshwater shells of a subglobular or conical form, 

 usually covered with a greenish periostraca, and sometimes ornamented 

 with bands, as inj^l- T^^fig. 95, which represents the shell of P. vivipara 

 of Europe and the United States, but the animal is incorrectly drawn, so as 

 to resemble a land snail. The foot is a large, flat body, Avith the sides parallel, 

 truncated in front and rounded behind ; the tentacles subulate, not annu- 

 lated, and bearing the eyes upon an enlargement of their external side. 

 Paludina decisa is about half an inch long, of a fine green color, and is 

 widely distributed in the United States. The animal is figured in the 

 Freshwater Univalve Mollusca of the United States. 



Amnicola resembles a minute Paludina in the form of the shell, but the 

 operculum is subspiral, and not concentric as in that genus. 



Valvata is a genus of small shells much like Amnicola, but the aperture 

 is circular and the operculum concentric. The branchiae are exserted, and 

 in the form of a minute plume. An exserted organ like a single thread is 

 supposed to be an accessory branchia. 



Paludina., Valvata, and J. w^7wZ^«>'/a, from Lamarck's fiimWjPerisfomata ; 

 and Cuvier and Swainson placed them under a division of which Turbo is 

 the type. 



Melania is the type of the sub-family Melaniinas, which includes a number 

 of genera, among which are Melania, Melanopsis, Pirena, and Anculosa. 

 In Melania the shell is solid, turreted, with a greenish or blackish 

 periostraca, the aperture elliptical, the external margin sharjD, and the 

 operculum corneous and subspiral. The species live upon calcareous 

 rocks, or upon the ordinary bottom of the rivers, especially in those 

 of the United States. The zoological characters of the genus are 

 given in the American Journal of Science, 1841, vol. xli. p. 21. 



Leptoxis (or Anculosa) is allied to Melania, but the shell is shorter. The 

 animal at<||!hes itself to rocks where the current is very rapid, and it seldom 

 moves from its position. The history of this genus is given in Chenu's 

 Illustrations Conchy ologiqiies, where it is illustrated by 170 figures. The 



287 



