FIELD BOOK OF PONDS AND STREAMS 



the brood pouch is formed by the inner gill and the number 

 of young is comparatively small, from two to twenty varying 

 with the species. The breeding season seems to continue 

 more or less through the year. 



Members of this family live in lakes and rivers as well as 

 in ponds and pools and small streams (Fig. 22). They live 

 on bottoms of sand and mud or clay and often creep up over 

 plant stems. They are widely distributed, almost sure to be 

 found in any kind of fresh water. Species are numerous and 

 difficult to distinguish. 



Pig. 266. — Sphere-shells, Sphcerium: i,S. sulcatum, 

 left valve, showing small hinge teeth {h)\ 2, same 

 shell with centrally placed beaks (a) ; 3, animal of 

 S. striatum, showing divided siphon (after Baker). 



Sphere-shells, Sphaerlum. — Mussels of this genus have oval 

 shells and centrally placed beaks (Fig. 266) ; the hinge teeth 

 are very small. The incun-ent and excurrent siphons are 

 separate except at the base. The unusually large species S. 

 sulcatum is often found in moderately swift streams. East 

 of the Rocky Mountains. 



Fig. 267. — Shell of Musculium showing the cap- 

 like beaks: I, left valve; 2. inner surface of right 

 valve, hinge teeth hardly developed (a) ; 3, shell with 

 cap-like beaks. 



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