9 



Spec. Char. A longituclinal, equalvalved, cu- 

 neiform, bivalve, open at the larger end ; 

 hinge lateral, without teeth, Avith a margi- 

 nal, partly internal, linear cartilage. Ani- 

 mal attached by a byssns. Muscular im- 

 pression nearly central. 



J\h there is no danger of confounding any other genus 

 with Pinna, no difficulty will occur on that head, 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 Linnean genera 

 in which it has been found needless to form any division. 

 The general form is an acute isoceles triangle, with the 

 shorter side rounded and gaping, and the acute angle 

 sometimes truncated : it is described by Linneus as 

 siibbivalvis 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 

 edfijes. The fibrous structure of the outer coat, which 

 extends far beyond the inner pearly one, has become ia 

 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 a,"enus Perna, however, 

 this structure is more remarkable from the length of th® 

 fibres, 



PINNA tetragoiia. 

 TAB. CCCXIII.-^-F/^. 1. 

 Spec. Char. Narrov/, smooth, or obscurely 

 ribbed, prismatic ; valves longitudinally 

 carinated with a suture in the middle. 

 8yn. Pinna tetragona. Brocchi, II. 589. 



P. subquadrivalvis ? Lata. Hist. Nai. 

 VI. 134. 



About twice as long as wide, with many slightly ele- 

 vated ridges chiefly placed above the suture ; section 

 nearly square, but varir.bh\ 



Tlse imperfections in the specimens describsd by 



