U. S. B. F.— Doc. 920. 



FIG. 13.— UPPER, Mytilus californianus, THE LARGE SEA MUSSEL; LOWER, 

 Mytilus edulis, THE SMALLER SEA MUSSEL; EACH THREE-FOURTHS 

 NATURAL SIZE. 



Both s]iecies are very common along the Northwest coast. 



FIG. 14. — PORTION OF SURFACE OF A ROCK NORTH OF NETARTS BAY 

 COVERED WITH Mytilus californianus. 



