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 wliich 

 the valves are inseparably united ; the substance of the 

 shell fragile, provided with a bi/sus or beard; gaping at 

 the larger end. Plate 5, fig. 1. 



This which forms the connecting link between the Bivalve 

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

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

 are united, is of a different 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 Pinnae are all inhabitants of the ocean ; in <hc sand 

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

 obtained standing erect, or affixed by their beards to rocLs 

 9hd stones. 



*** Univalves; Shell of one entire jnece. 



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 



