69 



MACTRA. 



(Plate VI. fig. 2.) 



Shell inequilateral, equi valve. Hinge having the 

 primary tooth complicated, with an adjacent sinus, 

 lateral ones remote and mutually inserted, (fig. 1.) 

 Shape various, sub-triangular, or broader than 

 long, sometimes gaping. The form of the hinge 

 is very remarkable. The compound middle tooth 

 and the adjoining hollow, filled with an elastic car- 

 tilage connecting the two valves, are easily to be 

 distinguished, when their construction has once 

 been attentively observed. We may here remark^ 

 that whenever an internal cartilage does occur, it 

 is generally of a triangular shape, or of one tending 

 to a three-sided prism, the acute angle of which is 

 placed under the apex of the shell. It is not pos- 

 sible to conceive a conformation more beautifully 

 adapted to the motion of the valves and the arti- 

 culations of the hinge. This cartilage, it appears, 

 should be considered as performing some other 



