BIVALVES PINNA. 75 



PINNA — Fin Shell, Nacre, or Sea-Wing. 



DESCRIPTION OF PLATE XVII. 

 Div, I.^Fig. 1. P. pectinata. Fig. 2. P. cancellata. 



Shell bivalve, fragile, upright, gaping at one end, and furnished 

 with a byssus; hinge without teeth, the valves united into one, 



THE number of species contained in this genus is 

 twenty-one ; they are in general very brittle and fragile, 

 and some are so similar to each other that they are with 

 difficulty distinguished. 



The usual form of the Pinnae resembles that of the 

 larger species of Muscles, being long and tapering, nar- 

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

 siderable breadth towards the opposite extremity: in 

 a few instances the form is rather compressed, and some- 

 times subangular. They universally gape at one end. 



The hinge is invariably without teeth, the valves, ne- 

 vertheless, adhere so closely in the region of the 

 beaks, that they appear as if united together. 



The Pinnoe are usually covered with longitudinal 

 ribs, and elevated transverse striae, often terminating in 

 imbricated scales, and prominent tubular spines, as may 

 be particularly observed in theP.rudis andP.muricata, 

 but in the younger shells of these two species, the spines 

 appear only as minute prickles ; in other specimens, as 

 the P. saccata, &c. the ribs are less articulated, and per- 

 fectly free from scales or spines. 



Some of the young shells of this genus are less 



