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, 



