«5 



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 un- 

 questionably derived from the habits of the ani- 

 mal, 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 de- 

 finition, that part of the margin to which the liga- 



