106 INVERTEBRATA OF MASSACHUSETTS. 

 FAMILY NAIADES, LAM. 



Shells flwoiatile ; hinge having a simple or divided, furrowed, cardinal tooth, 

 with or without a lateral tooth extending along the margin ; and sometimes 

 destitute of teeth ; muscular impressions compound. 



The shells embraced in this family are familiarly known by the 

 names of fresh-water clams or mussels. They inhabit 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 any thing very attrac- 

 tive ; 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 for- 

 eign collectors than the American Naiades. 



The teeth, when they exist, are strong, pyramidal, or com- 

 pressed, 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 occasion- 

 ally 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 developernent in such a continued 

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

 the Naiades might be reduced to a single genus. 



