MOLLUSCA. 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 
alge. The family contains a number of sub-families, the limitse of which 
are not well ascertained. elania amarula (pl. 75, fig. 90). 
Littorina is a genus of small marine shells which inhabit the coasts. 
L. 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 pl. 75, 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 Cerithiide. 
The mantle of Scalaria (pl. 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 in pl..75, jig. 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, with 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. 
Ammcola resembles a minute Paludina in the form of the shell, but the 
operculum is subspiral, and not concentric as in that genus. 
Valwata is a genus of small shells much like Amnicola, but the aperture 
is circular and the operculum concentric. The branchiz 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 Ampullaria, trom Lamarck’s family Peristomata ; 
and Cuvier and Swainson placed them under a division of which Turbo is 
the type. 
Melania is the type of the sub-family Melaniinee, 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 sharp, 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. 
Lepto«is (or Anculosa) is allied to Melania, but the shell is shorter. The 
animal attaches 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 
Lliustrations Conchyologiques, where it is illustrated by 170 figures. The 
287 
