60 WEST AMERICAN SHELLS 
often torn off by the waves and washed ashore. 
They are easily identified, though a. novice often 
confounds them with limpet shells. I have some 
exceedingly beautiful specimens that were taken 
from the piles that had been pulled up from a dis- 
used wharf at San Pedro. 
Chama exogyra, Conr., the Reversed Chama, has 
a coarser, more opaque shell, with little beauty of 
form or color, being usually of a dirty white. The 
chief difference, however, is seen in the curve of the 
umbones. If you stand a specimen of this species 
on its edge, with the beaks uppermost and curv- 
ing towards you, the side which was attached to 
the rock will be towards your left hand. But if 
you place a specimen of pellucida in the same posi- 
tion, the rocky side will be towards your right 
hand. I have gathered the two species growing 
side by side, and I have never known this rule to 
fail. 
Chama spinosa, Sby., the Spiny Chama, is a 
southern species, living mostly along the Mexican 
coast, but perhaps reaching as far north as San 
Diego. It resembles the first species, but its ridges 
are broken into close, short spines. 
We now come to the interesting family of the 
Heart-shells, or Cockles, as they are called in many 
places. There are over two hundred species of 
them known in different parts of the world, and 
they vary exceedingly in size and appearance. They 
are very decidedly heart-shaped, whether you look 
at them from the front or the back of the shell. 
Most of them have prominent ribs running from 
