47 
BIVALVES. 
MYA. 
(See Plate IV. Fig. 2.) 
Shell mostly gaping at both ends : hinge seldom 
with more than one tooth, solid, thick, and spread- 
ing, not inserted into the opposite valve. (Fig. 1.) 
The shells of this genus are, in shape, chiefly 
broader than long, and generally smooth, or slight- 
ly striated. The hinge, in some species, is con- 
nected internally by an elastic cartilage, which in 
the one valve is sunk in a triangular sinus, and in 
the other is protected by the broad tooth. In others 
the cartilage is wanting, and the teeth are more or 
less complicated ; sometimes they are crenate, or 
lobed, and one or more of the lobes, seldom the 
principal one, are slightly inserted. This genus is 
rather to be distinguished by the size and thick- 
ness of the teeth, than by any assignable form of 
them, in which they differ much. 
In a few instances the hinge is destitute of teeth, 
