It is characteristic of all the families of Mollusca lately noticed, that they live habitually concealed in burrows of their own making. These burrows have, however, been commonly perforations in a yielding medium, such as sand or mud, easily and rapidly made by the animal, and obliterated as soon as relinquished.
The species of the family last described occasionally penetrate more resisting substances, such as clay, and even chalk. But I have now to describe genera which have the power of perforating the hardest and most solid rocks. Some species, however, both of this and of the succeeding family, though decided stone-borers, occasionally content themselves with burrows in the softer materials.
In the family before us the shell is equal-valved, but very unequal-sided, the valves usually gaping, and connected by a hinge, which varies exceedingly, sometimes being merely rudimentary, at others having cardinal teeth. These latter, however, when present, are in some species lost under certain conditions of growth.
The animal is oblong or club-shaped, with siphons capable of great elongation, and united through their whole length. The mantle is closed except to give existence in front to a minute foot.
In the genus Gastrochæna, represented by one native species, the gallery perforated in the stone is lined with a deposit of calcareous substance, which forms a shelly tube frequently projecting from the stone, and serving for the protection of