THE CLAM AND OTHER BIVALVED SHELL-FISH. 55 



substance called the "ligament" (Fig. 58, liy). When the 

 clam is alive this ligament is slightly elastic, so as to allow 

 the valves, aided by the two great muscles within, to open 

 and shut. The tooth in the one valve and the cavity in 

 the opposite valve, together with the ligament, form the 

 "hinge" of the shell. The edges of the valves are also 

 thickened to support the hinge, and this side of the shell is 

 called the upper or dorsal side ; hence the opposite edge of 

 the shell is called the lower or ventral side. Thus the clam- 

 shell has an upper and lower side, and a front and hinder 

 end, and each side is for the most part like its opposite. 

 We shall also see that the animal within has a dorsal and 



FIG. 58. iDside view of the clam-shell (natural size). P7, pallial line, made by 

 the mantle; acini', impression of the anterior; adin", of the posterior adductor 

 muscle; liy, ligament; bk, beak. 



ventral side, a head-end and hinder-end, and that both 

 sides of the animal are alike. 



Moreover, we notice over the hinge, outside of the shell, 

 that each valve is full and swollen over the site of the 

 hinge. Each swelling is somewhat like a bird's beak ; hence 

 they are called collectively "the beaks." 



We also observe that the surface of the valves is rough and 

 marked with lines nearly parallel to the outer edge of the 

 valves. These are the lines of growth. They are not at 

 equal distances apart, for the clam probably does not grow 

 steadily at all times of the year, but by fits and starts. If 

 we pick up some very young clams, however, we shall see 



