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DIMYARIA.—GASTROCHÆNIDÆ.

hinge furnished at one period of life with cardinal teeth. The ligament is external and projecting. The animal is oblong, with the mantle closed, except in front, where exit is afforded for a minute finger-like foot, furnished with a byssal groove. The siphons are united, and their orifices, which are large, are margined with a fringe of simple filaments.

SAXICAVA.

"The Saxicavæ," observe the authors of the British Mollusca, "are borers, although the habit of boring does not seem necessary to their existence, since we find them very commonly free. If there be a crevice, however, in rock, shell, coral, or sea- weed, into which they can thrust themselves, they do so; and if near a limestone rock, perforate it, and form crypts, in which they live. Mr. Osier states that, when young, they are very active