Page:Natural History, Mollusca.djvu/275

Rh 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 brilliant; 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, except behind the hinder edges, forming, when in conjunction, two tubes, the larger of which is guarded 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 inhabiting 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.