BIVALVES MYTILUS. 4? 



PINNA.— Fin Shell or Nacre, also Sea- Wind. 



Animal— a Limax: Shell bivalve, fragile^ nprighty gaping 

 at one end, and furnished ivith a hyssits or heard; hinge 

 without teeth, the valves united into one* 



The number of species contained in this genus is limiteri 

 to eighteen, and some of those are so ahke, as barely to 

 admit of being called distinct. 



The usual form of the Pinnae is something allied to that 

 of the larger species of Muscles, being long tapering shells, 

 narrow at the beaks, and gradually expanding to a con- 

 siderable breadth to the opposite extremity: however, 

 there are instances where Uie form is more compressed. 



The Pinnae ai-e by no means such entire or solid shells 

 as the Muscles, for they are (with few exceptions) exceed- 

 ingly brittle or fragile in substance, and almost invaria- 

 bly gape at one end. The greatest portion of the Pinnae 

 have longitudinal ribs, and on them are placed elevated 

 transverse striae, often terminating in imbricate arched 

 scales, and prominent canaliculate tubular spines. 



Such is the case with the Pinna Rudis and Muricata: 

 in the younger shells of these two species, the spines ap- 

 pear but as minute prickles. In other specimens, as the 

 Pinna Saccata (Satchel), &c. &c. the ribs are not so 

 articulate, but more undulate, and are perfectly free from 



