26 WEST AMERICAN SHELLS 
length of about four inches. They have even been 
found in streams of the northern Sierras, up to the 
height of 5,000 feet. They also live in Oregon, 
Washington and the Hast. 
The shell is oblong in shape, with the umbo 
near one end. There is a strong, prominent hinge- 
tooth, also distinct muscle-scars and an unbroken 
pallial line. The interior of the shell is pearly and 
deeply tinted, usually purple but sometimes blue 
or salmon-colored. The outside is covered with a 
dark, horny epidermis, usually much eroded at the 
umbones. The river-mussels do not burrow 
deeply, but are only partly buried in the sand. 
They love quiet stretches where the current is not 
too strong, and where there is but little danger that 
they will be overwhelmed with sand or mud. They 
multiply rapidly, though many of the young ones 
perish after they are thrown out into the current 
to shift for themselves. Those that survive begin 
to build shells, and when they are once established 
their growth is rapid. M. falcata, Gld., is a variety 
of the above species. 
The Anodontas belong to the same great family, 
but are destitute of hinge-teeth, as their name 
indicates. 
Anodonta nuttalliana, Lea, Nuttall’s Anodon, 
is shown in Figure 12. This river-shell is very 
thin, of bluish pearl within, and is covered with a 
greenish-brown epidermis. The hinge line is pro- 
longed obliquely upward, forming a nearly right 
triangle above the oval part of the shell. Its length 
is three inches. The specimen from which the 
