ZOOLOGY. 



255 



waters, are found on the surface, or immediately under the 

 mud, some are used as food, and often produce small 

 pearls. 



17. Pinna, Nacre. Inhabitant a Limax> 



Shell of two parts, connected by a toothless hinge, by which 

 the valves are inseparably united ; the substance of the 

 shell fragile, provided with a bijsus or beard; gaping at 

 the larger end. Plate 5, fig. 1. 



This which forms the connecting link betvyeen the Bivalve 

 and Univalve shells, is so formed as to possess equal claims to 

 either division ; the suture or hinge by which the two parts 

 are united, is of a dilferent substance to the shells, but is 

 hard and not flexile ; of the byssus (a sort of silk) is ma- 

 nufactored various articles of apparel, and Manufactories 

 are established for this purpose at Naples and Palermo. 



The Pinna? are all inhabitants of the ocean ; in the sand 

 and mud on the shore of sheltered bays, they may be often 

 obtained standing erect, or affixed by their beards to recks 

 and stones. 



Univalves; Shell of one entire piece. 

 Division I. 



WITH A REGULAR SPIRE. 



The almost imperceptible gradation, in this beautiful Class 

 of Animals, renders the present mode of classification pecu- 

 liarly interesting- as we are not under the necessity of 



