Page:Natural History, Mollusca.djvu/267

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fined localities it is often much distorted, but free specimens are very regular."[1] The epidermis in this species often projects beyond the edges of the valves, in the form of a ragged fibrous fringe.

Family Mytilidæ.


The members of this family are almost all marine, though one genus (Dreissena) inhabits the rivers of Eastern Europe, and is now found plentifully in some of the navigable streams of this country, having been introduced, as is supposed, adhering to timber, or to the bottoms of ships. The distinctive characters of the family, as enumerated by M. de Blainville, are as follows:—

The shell is regular, with the valves equal but much lengthened, and produced on one side. The hinge is destitute of teeth; the ligament dorsal and linear. The texture of the shell is sometimes thin and horny, and it is often covered with an epidermis. There are two muscular impressions, the hinder of which is considerably larger than the other.

In the animal we find the mantle adhering towards the borders, but open throughout its inferior portion, with a distinct anal tube, but the branchial one rudimentary, and indicated only by a thickening of the posterior borders of the mantle. The foot is narrow and tongue-shaped, grooved down its centre, and furnished with a byssus backwards at its base. In some of the genera, as Lithodomus a foreign group which burrow in the hardest rocks,

  1. Forbes and Hanley.