65 
M A C T R A. 
(Plate VI. Fig. 2.) 
Shell inequilateral, equivalve. 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 
cartilage 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 possible to conceive a con- 
formation more beautifully adapted to the motion 
of the valves and the articulations of the hinge. 
This cartilage, it appears, should be considered as 
performing some other functions than those of the 
