64 WEST AMERICAN SHELLS 
ally it has many synonyms. Its variety occiden- 
tale, Newe., is found in a little brook running out of 
Mountain Lake, near San Francisco. The sheli 
is about one-eighth of an inch in length. 
Pisidium compréssum, Prime, the Compressed 
Pea-shell, is rather unfortunately named, for it 
is plump and full, of the shape and size of a radish 
seed. It is covered with a brown epidermis, and 
is found in Washington and also in Arizona. 
Pisidium ultramontanum, Prime, the Western 
Pea-shell, is still smaller than the last species. 
Shell thin, smooth, ight brown. From Utah, also 
from Dueck Lake, Lassen Co., and Feather River in 
Plumas Co., Cal. 
Pisidium rowélli, Sterki, Rowell’s Pea-shell, is 
a species with a translucent, rather thin shell, 7.5 
mm. long, collected near Sisson, at the base of Mt. 
Shasta, by Rev. J. Rowell. 
Pisidium ashmunt, Sterki, Ashmun’s Pea-shell, 
has a minute, horn-colored shell, less than 3 mm. 
long. From San Rafaels, New Mexico. 
Pisidium idahoénse, Roper, the Idaho Pea-shell, 
has a large shell, with beaks scarcely raised, ex- 
terior yellow and glossy, interior bluish white, 
length 8mm. From near Old Mission in Northern 
Idaho. Also from Seattle, collected by Mr. P. B. 
Randolph in 1895. 
Figure 50 gives a good representa- 
tion of a typical species of the next 
genus, Sphaérium sulcatum, Lam., the 
Furrowed Sphere-shell. The mollusks 
of this genus are closely allied to those 
Fig. 50 
