THE BIVALVE SHELL. 109 



PLATE XIII. 

 The Bivalve shell, and its parts. 



Some of the principal lineaments of a Bivalve shell 

 are the dorsal, ventral, anterior, and posterior margins ; 

 pibs, ridges, and spines; the umbones; the 

 ligament; the linula; projecting portions known as 

 the ears ; the accessory plates ; and the 

 periostracum. 



The Margins determine the contour of the valves. 

 They are clearly shown in fig. 8. 



The Ribs radiate from the umbones to the anterior, 

 posterior, and ventral margins, figs. 4, 4a, plate XIV. ; the 

 Ridges are arranged in concentric form, and coincide 

 with the lines of growth, they can be seen in fig. 1 ; 

 Spines vary greatly in length, thickness, and shape, some 

 are slender and pointed, others narrow, thin, long, flat, or, 

 plicated, and project at various angles. The Umbones, 

 fig. 1. The umbo of each valve originates as the 

 embryonic shell, and forms the point from which the 

 growth of the valve proceeds. 



The Ligament, figs. 1, 3, is an elastic structure, 

 consisting of two parts, an external horny substance, and 

 an internal substance of very similar character, known as 

 the Cartilage, both of which act as a hinge, and hold 

 the valves together. Figs. 3, 3a, and plate XTV. 



The Linula is a part usually oval in outline, in the 

 form of a depression in front of the umbones, fig. 1 . 



The Ears, figs. 3, 3a, 5, are known as anterior and 

 posterior. They are elongated portions of the dorsal 

 margin of the valve. 



Accessory plates are small valves which protect 

 the dorsal margin. In the common Piddock (Pholas 

 dactylus\ the number is five. 



