PINNA. Linn. 
Spec. Char. A longitudinal, equal valved, cu- 
neiform, bivalve, open at the larger end ; 
binge lateral, without teeth, with a margi- 
nal, partly internal, linear cartilage. Ani- 
mal attached by a byssus. Muscular im- 
pression nearly central. 
As there is no danger of confounding any other germs 
with Pinna, no difficulty will occur on that tread, but 
the distinction of the species is by no means easy, they 
so nearly resemble each other in form, it is one of 
those perfectly natural and insulated JLInnean genera 
in which it has been found needless to form any division. 
The general form is an acute isoceies triangle, with the 
shorter side rounded and gaping, and the acute angle 
sometimes truncated : it is described by Linneus as 
suhhivalvis in consequence of the close connexion of the 
valves by the linear cartilage, a connexion rendered so 
close often by being on the hollow side, as to prevent 
all motion of the valves, which in one species even 
(p. saccata) sometimes grow together at the opposite 
edges. The fibrous structure of the outer coat, which 
extends far beyond the inner pearly one, has become in 
a manner proverbial ; but the outer coat of other shells^ 
even univalves, is generally composed of perpendicular 
fibres, although not often of so coarse or loose a texture, 
and seldom extending far beyond the inner laminated 
compactor pearly coats. In the genus Perna, however, 
this structure is more remarkable from the length of the 
fibres. 
PI N N A tetragona. 
TAB. CCCXIIL— Fig. 1. 
Spec. Char. Narrow, smooth, or obscurely 
ribbed, prismatic ; valves longitudinally 
carinated with a suture in the middle. 
Syn. Pinna tetragona. 13 race hi, II. 589 . 
P. subquadrivalvis ? Lam. Hist . Nat. 
VI 134. 
About twice as long as wide, with many slightly ele- 
vated ridges chiefly placed above the suture ; section 
nearly square, but variable. 
The imperfections in the specimens described by 
