Page:Natural History, Mollusca.djvu/302

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DIMYARIA. — MYADÆ.

ing in it the Solenidæ already described, as well as the genera of which Messrs. Forbes & Hanley constitute the three families Myadæ, Corbulidæ, and Pandoridæ; the British representatives of the last-named two are few, and for the most part small, though remarkable for the curious form of the shell, which projects at one side as if distorted.

The shell in the Myadæ is nearly regular and equal-valved, oblong, and somewhat coarse in appearance, gaping at the two extremities. The hinge is incomplete, the teeth being gradually effaced, but generally composed of one or two oblique diverging folds. The ligament is sometimes internal, sometimes external. There are two distinct muscular impressions connected by a pallial impression, widely sinuated behind. The animal is compressed, becoming in the different genera more and more cylindrical, as well as more completely closed, and prolonged backwards into two long siphons, which are ordinarily united, with fringed extremities. In front, the mantle allows the protrusion of a very small foot. The gills are narrow, free, and prolonged into one of the siphons.

All the animals of this family live plunged in the mud or sand, which is ordinarily covered by the sea. They rarely change their place or their position, invariably being vertically immersed, with the siphons above and the foot beneath. Many of the species grow to a large size, and as they are both wholesome and palatable, they are on some coasts much prized for the table.


Genus Mya.

The hinge in this genus is composed of one or two oblique folds, diverging backwards from a