73 



PINNA. 



(Plate XI. Fig. 1.) 



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

 ing 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 

 connected 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 Linnoean de- 



