102 CLASS ACEPHALA. 
It rem:ins half buried in the sand, and anchored by its 
byssus. In the 
ARCA, Lin., 
The valves are equal and transverse, that is to say, the hinge 
occupies the longest side. It is furnished with a large num- 
ber of small teeth, which interlock with each other, and, as 
in the subsequent genera, with two bundles of transverse and 
nearly equal muscles inserted into the extremities of the 
valves, which seem to close them. In 
ARCA, (properly so called) Lam., 
The hinge is rectilinear, and the shell most elongated in a 
direction parallel to it. ‘The summits are generally convex, 
and curve over the hinge, but are separated from each other. 
The valves do not close perfectly in the centre, because there 
is a horny plate or tendinous band before the abdomen of the 
‘animal that serves for a foot, and by which it adheres to sub- 
merged bodies. ‘They are found in rocky bottoms near the 
shore, and are usually covered with a hairy epidermis. 'They 
are not much esteemed for the table. 
Some species are found in the Mediterranean, and a great 
many fossil, in strata anterior to the chalk, particularly in 
Italy. 
Certain arce, in which the teeth of the two ends of the 
hinge assume a longitudinal direction, are distinguished by 
Lamarck under the name of CuCULL@A, (arca cucullata), 
Chemn. 
We ought also, it is probable, to separate the species with 
well marked ribs, and completely closing and interlocking 
edges, for we may presume that their animal is not fixed, but 
rather resembles that of a pectunculus. 
We have a still better warrant for removing the arca tor- 
