Page:Natural History, Mollusca.djvu/323

Rh laden with their effete particles, and to bring within the reach of their prehensile organs the animalcules adapted for their sustenance."

In this genus, which I select to represent the Class, the valves are unequal; and the lower one, more prominent than the other, is perforated to admit the passage of a short fleshy stem, by which the animal is firmly attached to rocks, and other foreign bodies. The shell is delicate in texture, more or less triangular, and symmetrical. The shelly frame-work of the arms, sometimes called by collectors the carriage-spring, is attached to the inner surface of the upper valve, or that which is not perforated.

Most of the species of this genus, as of the whole Class, are known only in a fossil state; those which are recent are widely diffused, and flourish in extremely warm and extremely cold climates, as well as those which are intermediate. Thus some are found in the Indian ocean, beneath the equator; while the T. psittacea, brought home from the late Arctic expedition, was dredged at Boothia in the Polar sea.