382 NORTH AMERICAN FAUNA 61, FISH AND WILDLIFE SERVICE 



Five species of Macoma were collected. 



Two small mussels, Musculus discors and M. vernicosus, were 

 collected. Each is about 10 to 15 millimeters long, and is reddish 

 brown or tan. The latter has a shining, varnished surface. 



The mud clam or gaper, Mya truncata, was collected once, at 

 Unalaska. 



The larger, abundant mussels are of two kinds. Mytilus edulis, 

 the edible or blue mussel, is smooth and regular and is purplish 

 blue to black in color with a bluish nacre (fig. 11). The umbo is 

 apical, unlike that of the horse mussel. The edible mussel is used 

 for food by the natives and is said to be best when there is a roll 

 of snow-white fat on either side of the body. When yellow and 

 lean, the flesh is unpalatable. Volsella modiolus, the horse mussel, 

 can be distinguished from the former by its larger, thicker shell 

 and by the presence of a brown periostracum. The umbo is never 

 at the apex, and the nacre is gray. The horse mussel usually grows 

 solitary or in clusters of a few, while the edible mussel may cover 

 the rocks in an area many feet in diameter. Both attach to rocks 

 by a thready byssus, but the horse mussel usually is partly buried 

 in sand. (A third large mussel, Mytilus calif ornicus , was collected 

 only once — at a depth of 30 fathoms off Sanak Island.) 



If TSBT^ 



r i 





Figure 11. — Edible or blue mussels, Mytilus edulis, in tidal zone. Unalaska 



Island, July 10, 1937. 



