66 WEST AMERICAN SSHEEES 
in length. It is somewhat triangular, its surface 
very smooth and bright, its external color white, 
or perhaps marked with brown, while internally it 
is white with a purple stripe at the posterior end. 
It was called Psephis tantilla, in W. C. S., and it 
has also received various other names. It ranges 
from Sitka to Mexico. 
Figure 51 represents a very large, heavy shell 
now known as Tivela stultorum, Mawe, the Great 
Tivela. Of course that is not a correct translation 
of the Latin; that you may do for yourselves, only 
avoid the joke; but as this is the largest and finest 
species of the genus we may well call it ‘‘Great.’’ 
It was called Pachydesma crassatelloides in W. C. 
S., and it has also had a good many other names, 
all of which have been duly considered at the 
