148 WEST AMERICAN SHELLS 
vegetable eating mollusks, and clear away much 
matter in the water which would otherwise go 
to decay. Their shells are generally covered with 
a greenish brown epidermis, to protect them from 
the effects of acids which may be present in the 
fresh water. 
ably eroded. 
Sometimes the beaks are consider- 
Iimnea stagnalis, WLinn., the © 
Pond Limnea, Figure 127, is the 
largest of this class of fresh-water 
mollusks, and is universally dis- 
tributed, both in this country and 
in the Old World. It is a very dis- 
tinct species, and can be instantly 
recognized. Spire very slender, 
body-whorl and aperture very large; 
shell thin and delicate. It grows 
sometimes to a length of nearly two 
inches. This mollusk inhabits lakes and rivers, 
and is found on the Sierras, in Utah, and in many 
other localities. 
Iimnea palustris, Miull., the Marsh 
Limnea, is shown in Figure 128. It is 
found all round the world, in the north- 
ern hemisphere. The whorls are rounded, 
five or six in number, and the aperture 
is shorter than the spire. The shell is 
horn-colored, and its form is well repre- 
sented in the figure, which is larger than 
the average specimen. ‘There are many 
varieties of this widely distributed species. 
