294. . DIMYARIA.—GASTROCH ANID. 
FAMILY GASTROCH ANID. 
(Stone Borers.) 
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 occasion- 
ally 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, how- 
ever, 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 exceed- 
ingly, sometimes being merely rudimentary, at 
others having cardinal teeth. These latter, how- 
ever, 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 Gastrochena, 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 
