SNAILS AND MUSSELS 



Musculium. — The shells of Musculium are thinner than 

 those of SphcBrium; the hinge teeth are small, almost missing, 

 and in nearly all of the species the beaks are cap-like (Fig. 267). 

 The siphons are longer than those of SphcBrium and are at- 

 tached to each other throughout their length. Length of the 

 shells, three-eighths of an inch. Musculium and SphcBrium 

 live in the same kind of haunts. 



Fig. 268. — Pill clam, Pisidium: i, P. virginicum, 

 right valve, note the long anterior end; 2, same, left 

 valve from within, showing cardinal teeth (a) ; 3, 

 same, shell showing beaks (6) located posterior to the 

 center; 4, animal of P. pohtum decorum, showing 

 the single siphon (after Baker) . 



Pill-clams, Pisidium. — The valves of the shell are slightly 

 unequal in size; there is one cardinal tooth in the right and 

 two in the left valve (Fig. 26S). The foot is tongue-shaped 

 and large for the size of the animal. The siphon is short and 

 only the excurrent part is present; water enters the mantle 

 cavity through a cleft in the mantle. Like the other small 

 mussels the members of this genus live in varied habitats. 

 They not only burrow in mud but they crawl up on plants; 

 Elodea, Potamogetons, and floating algae frequently hold many 

 of them. Pisidium virginicum is recognizable by its size, 

 unusually large for Pisidium, its oblique shell and peculiar 

 cardinal teeth. Length of shells, one-quarter to three-eighths 

 of an inch. 



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