brittle that it is hard to grind satisfactory sections. A cuticular 

 gland is present along the margins of the mantle (fig. 26, cgl.), 

 and in this gland fragments of a weak layer that no doubt is 

 cuticle is present but the frequent removal of the mantle margin 

 far into the interior of the shell would seriously interfere with 

 the formation of a continuous cuticle. As might be expected in 

 an animal where so large a portion of the mantle is movable, the 

 nacre is" not formed of such uniform delicate layers as are found 

 in some shells. Layers are present but the carbonate of lime 

 seems to have become partly crystalized and the layers inter- 

 rupted in places. 



The shell material may vary in color considerably in individ- 

 uals, or in the same individual, but this seems to be a variation 

 in the nacre and not caused by the presence of other layers. 

 The variation is especially well marked in the smaller more 

 southern scallop, Pecten irradians, where the outer portions of 

 each of the ridges that are so prominent in this species is deeply 

 colored while the inner surface of the ridges and the portions 

 forming the valleys between the ridges are white. It all seems ' 

 to correspond in structure to nacre however. 



The valves are attached to each other along the hinge line by 

 a very weak ligament which represents a modification of a 

 cuticle that serves simply to hold the two valves together along 

 this line. Near the middle of the hinge line, just beneath the 

 beaks, there is in each valve a deep and broad somewhat tri- 

 angular depression that in old shells opens quite broadly on the 

 hinge line. This depression, (fig. 5, cp.) forms the cartilage 

 pit, into which is set the end of an elastic pad, the so-called 

 cartilage, (fig. 10, c.) which is a modification of the ligament. 



When the adductor muscle contracts and the valves are drawn 

 together, this pad of cartilage is compressed and serves, when 

 the muscle relaxes, to separate the valves again. The cartilage 

 is composed of the same material as the enlarged elastic liga- 

 ments of such forms as the fresh-water mussels, and no doubt 

 they have had a common origin. They both serve to open the 

 shell by their elasticity, in the one case by expanding after being 

 compressed and in the other by contracting after being stretched. 



II 



