FRESHWATER MUSSELS. 263 
these Mollusks in the large rivers affords peculiar 
facilities for observation, has stated that under 
peculiar circumstances certain species do spin a 
byssus, both in the young and adult conditions. * 
The shell is lengthened oblong, equal-valved, 
unequal-sided, though in a much less degree than 
the family last noticed. ‘The interior is pearly, 
often very brillant; the exterior is invested by a 
horny epidermis, which, though generally black or 
olive, is sometimes richly coloured. The hinge 
varies greatly, being sometimes destitute of teeth, 
and sometimes furnished with long ones. The 
muscular impressions are numerous, the ligament 
and cartilages are external. 
The animal has the mantle free all round, ex- 
cept behind the hinder edges, forming, when in 
conjunction, two tubes, the larger of which is 
euarded by pointed and tooth-like tentacles. The 
foot is large, compressed, somewhat four-sided. 
These Mollusks live at the bottoms of rivers and 
lakes, plunged perpendicularly into the mud, with 
the terminations of the siphons just exposed. They 
sink deeper when disturbed. Sometimes they are 
found under the shelter of stones in rivers with 
rocky bottoms. 
The distribution of the family is peculiar. Very 
few are found in the Eastern Hemisphere, but all 
the rivers of America swarm with them. More 
than two hundred species are enumerated as in- 
habiting the United States. 
The sexes of these animals are distinct, and may 
be distinguished by the shell in the female being 
more swollen than that of the male. 
* Ann. Nat. Hist. vi. 
