166 UNIONID^. 



Animal Avith the mantle adhering between the two openings, of 

 which the upper one is plain and the lower fringed ; foot very large ; 

 gills long, sub-equal, united posteriorly. 



The shells embraced in this family are familiarly known by the 

 names oi fresh-water clams or mussels. They inhaliit most, if not 

 all, of our collections of fresh water, whether still or of rapid flow. 

 Their aspect is peculiar, and there is such a stamp of identity upon 

 them as forbids their being confounded with any other family of 

 shells. Exteriorly they seldom present anything very attractive ; 

 but no one can fail to admire the beautifully tinted pearl of their 

 interior. The few species inhabiting New England are simple and 

 unpretending in their appearance ; but our western waters furnish 

 species infinite in the variety of their shapes, colors, and marking ; 

 and no shells are more eagerly sought for by foreign collectors tlian 

 the American Naiades. 



The teeth, when they exist, are strong, pyramidal, or compressed, 

 and appear as if they had been abruptly fractured at their tips. 

 The beaks of mature shells are almost always found to be eroded, 

 either by the gravel or other substances which are washed over 

 them, or by some chemical process. The foot of the animal is 

 tongue-shaped, and serves to perform no inconsiderable journeys. 

 In quiet water, where there is a layer of mud at the bottom, the 

 furrows, traced by dragging the shell along on its sharp edge, are 

 readily seen. 



All the species are capable of producing pearls ; and occasionally 

 some of no inconsiderable beauty and value are found. Old and 

 deformed shells are most likely to contain them ; and in fact they 

 seem to be the products of injury or disease. 



The animal, in all the genera included in this family, seems to 

 have the same organization ; and the teeth, on the arrangement of 

 which the genera are founded, are observed to dwindle from their 

 greatest number and fullest development in such a continued series, 

 until they wholly vanish, as to lead to the belief that all the Union- 

 idcB mio-ht be reduced to a sino-le o'cnus. 



[The young are found to attach themselves by a byssal thread, 

 like the Mytilidw, which this family every way resembles. The fe- 

 males are distinguished by being much more full and somewhat 

 pouched at the posterior ventral margin ; and the embryonic young 

 are carried in the folds of the outer gills. Rafmesque and Swain- 

 son have proposed many generic divisions, and others founded on 



