73 



PINNA. 



(Plate XI. fig. 1.) 



Shell sub-bivalve, brittle, erect, gaping, throwing 

 out a beard or byssus. Hinge toothless, the valves 

 being inseparably united. 



Shape broad at one end, and gradually tapering 

 towards the other. Valves convex, equal, and con- 

 nected on the side of the hinge by a membrane, in 

 such a manner as to form in fact an univalve shell 

 bearing the appearance of a bivalve. The valves 

 are incapable of motion in their hinge, but are 

 liable to a forcible separation. 



In the one instance of the Pinna, the method of 

 Linnaeus in making the hinge, or that part nearest 

 the apex, the base of a bivalve shell, seems unques- 

 tionably derived from the habits of the animal, 

 which stands erect under water, infixed in the mud 

 by the smaller end of his habitation. But we may 

 doubt whether, according to the usual definition, 

 that part of the margin to which the ligament ad- 

 heres ought not to be considered as the hinge : if 

 so, the length of the shell will be less than its 

 breadth ', which is contrary to the Linn^an de- 



