INTRODUCTION. 



,'37 



Umbones. 

 These are the prominent points of the dorsal edge, where 

 the growth of the shell commenced, and aie called beaks, by 

 some English writers. In some instances they are close to 

 each other; in others they are rendered distant from each 

 other by the intervention of areas in the hinge, as in Spondyli, 

 &c. In Pectunculus they are straight; in Venus curved 

 towards the anterior margin ; in Isocardia, spiral ; in Chama, 

 decumbent ; in Diceras, free. In shells subject to external 

 corrosion, the process commences at the umbones. 



Fig. 75, distant ; 76, straight ; 77, curved; 78, spiral; 79, decumbent ; 

 80, free; 81, close. 



GENERAL CHARACTER OF BIVALVES. 



When the breadth is spoken of, the distance between the 

 most convex parts of both valves, when closed, is intended ; 

 but when an expression implying thickness is used, it refers to 

 the substance of each valve : it is important to bear this in 

 mind, as many persons have been misled by descriptions in 

 which the distinction has not been attended to. Glycimeris 

 (fig. 67 in the plates) is a thick shell, but Anatina (fig. 69 in 

 the plates) is a broad one. 



