MOLLUSCS. 



341 



The Melaniid^ are also fresh-water inhabitants, in which the shell is covered with 

 a thick epidermis, which, in some species, is so dark that the family name (mdas, 

 black) is very appropriate. Others are brown or dark green. The shell is usually long, 

 turreted, or conical, with a small mouth. The foot is large and triangular, the proboscis 

 short but stout, and the eyes are near the bases of the tentacles. The family is readily 

 separable into two sub-families, both on structural characters, and on geographical 

 distribution. The first, the Melaniinffi, are oriental, only a few being found on the 

 North American continent. In these, the aperture is usually broadly rounded and 



MJyd£M7i^ 



Fig. 432. — PaLudina vivipara. 



not produced in front, though often channelled or notched, while the margin of the 

 mantle is fringed, and many of the species are ovo-viviparous ; that is, the eggs under- 

 go their development and are hatched outside the parent. In the other 

 sub-family, the Strepomatinge, the margin of the mantle is plain, and the 

 eggs are laid and attached to stones and plants. With three or four ex- 

 ceptions, all the Strepomatinae are confined to the United States, a few 

 extending to the "West Indies. 



Of the MelaniinsB, the most prominent genus is Melania, which con- 

 tains about four hundred species, mostly distributed through Asia and 

 Polynesia, though a few are found in tropical America and southern 

 Europe. In many of the specimens the apex of the shell has disap- 

 peared, owing to the erosive action of the water which they inhabit. 

 Melanopsis costata, a common Syrian species of a genus allied to 

 Melania, is said to inhabit the Dead Sea, the only exception, so far as I am aware, 

 to an exclusively fresh-water habitat in the family. 



Fig. 4SS.—Pleur- 

 rocerapallidwm. 



