BIVALVES—— PINNA. 75 
PINNA.__F in Suez, NAacre, or Sea-Wina. 
DESCRIPTION OF PLATE XVII. 
Dw. 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 contaired 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 Pinne 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 Pinne are usually covered with longitudinal 
ribs, and elevated transverse striae, often termimating m 
imbricated scales, and prominent tubular spines, as may 
be particularly observed in the P.rudis and P.muricata, 
but in the younger shells of these two species, the spines 
appear only as minute prickles; im 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 
