SNAILS AND MUSSELS 



Family Pleuroceridae, Goniobasis. — These are gill-bearing 

 snails with an operculum. The shells have a long, conical, 

 right-handed spiral with 7 or more whorls (Fig. 256) ; full 

 grown ones are one and one quarter inches long. The 

 animals live in rapid currents, among water plants on lake 

 shores, in varied habitats. 



Family Valvatidae. — These snails (Fig. 257) are small, with 

 shells not more than a quarter of an inch wide, with promin- 

 ent keels or ridges on the outer surface. The animals have a 

 short siphon which is two-lobed in front. The gills are 

 extended out of the shell when the snail is moving; the left 

 one is carried like a plume; the right is only a degenerate 

 finger-like process. Found in lakes in both shallow and deep 

 water. Widely distributed. 



Fig. 258. — Vivipara contectoides, which has the 

 top-shaped shell characteristic of the family; shell 

 aperture filled by the concave operculum. 



Family Viviparidae. — The shells of these gill-bearing snails 

 (Fig. 258) are globular or top-shaped, moderately large but 

 not more than two inches long. The young snails are bom 

 alive. The species Vivipara contectoides has four brown 

 bands on the body whorl. Found in lakes and rivers on 

 muddy bottoms. 



Mussels. — Pelecypoda 



Mussels are the only other fresh water mollusks besides the 

 snails. They are familiar either as rather large, dark-shelled 

 mussels which burrow in muddy bottoms or as "finger-nail 



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