73 
PINNA. 
(Plate XL 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 Linneean de- 
