THE MUSSELS. 1 73 



these muscles, sharpened to an exquisite edge and point, the 

 Indians head their harpoons and other instruments for fish- 

 ing. They fasten them on with a kind of resinous substance. 



In large and old specimens the wrinkles are seen 

 only near the edge of the valves. 



Modiola fornicata, Cpr. , for-ni-ca'-ta, Arched Mus- 

 sel, has a very short and full shell, somewhat wedge- 

 shaped, having a breadth more than half of its length. 

 The naked shell is white, but it is usually covered 

 with a light-brown epidermis, especially near the 

 edges. It seldom grows to a length of more than an 

 inch. I have found it in Monterey bay, under stones, 

 attached by a byssus. 



Mytilus bifurcatus, Conr. , Mit'-i-lus bi-fur-ka'-tus. 

 The shell of this species is like that shown in Fig. 

 144, but it is without the internal shelf at the point 

 of the valve. It is very wide for its length, and is 

 somewhat curved. 



Mytilus edulis, Iyinn., e-du'-lis. This is the pur- 

 ple mussel which is so abundant on the shores of the 

 Atlantic. The shell is smooth and regular, and is 

 covered with a dark, glossy epidermis. Within, it is 

 of a rich purple color. It is found abundantly in 

 San Francisco bay, as well as elsewhere, clinging in 

 large groups to posts and wharves. Its length is 

 seldom more than two inches. 



So much for the marine mussels. Now we will 

 turn to their relatives which live in fresh-water lakes 

 and streams. 



Of these there are but a few species west of the 

 Roqky mountains, though to the east of that dividing 

 ridge, in the great Mississippi valley, the species are 

 numbered by the score and almost by the hundred. 

 The great investigator of these mollusks was Isaac 



