MACTRADA. 279 
outline is more triangular, and their surface is 
smooth, or merely marked with fine concentric 
lines. They are generally swollen; their sub- 
stance varies much in texture; they are often in- 
vested with a strong epidermis. ‘The valves are 
connected together by a hinge, consisting of a 
forked diverging tooth in one, raised on a liga- 
mental fulcrum lodged in a cavity, which is margi- 
nated in the other, a connecting cartilage and small 
external ligament completing the union.” The 
pallial impression closely resembles that of the 
Veneride. 
The animals of this family differ in the degree 
in which the mantle is open or closed in front ; there 
are two siphonal tubes, which are connected and 
fringed at their orifices, and sometimes studded 
with pointed warts. 
GENUS MACTRA. 
The shell is more or less triangular, slightly 
gaping, with a smooth or concentrically striated 
surface, covered with an epidermis. The hinge 
is composed of a V-shaped cardinal tooth in one 
valve, locking into a marginated pit in the other, 
and of a long lateral tooth on each side of the same 
valve, which also fits into a deep groove with tooth- 
like margins. The sinus of the pallial impression 
is shallow but wide. 
The animal has the mantle open as far as the 
siphons, its edges are fringed; the foot is strong, 
tongue-shaped and bent, capable of protrusion to a 
considerable extent. It is used, as in the Cockles, 
for burrowing in sand, which these animals do 
with great ease and rapidity. They are inhabi- 
