WATER SNAILS. 
215 
times the shell is laid flat, kept at an angle 
of 50° or 60°, or even raised so as to be perpen¬ 
dicular. When swimming along the surface of 
the water, the shell lies flat on the surface. 
Planorbxs spxrqrbis —[the Round-edged Goi 
Shell ) (Pl. XI., fig. 129)—The shell of this 
species is very closely allied to that of P. vortex , 
from which it differs in being thicker, less 
flat, the whorls rounder, and the keel less 
distinct. Its habits are similar to those of 
P. vortex . 
It is more widely diffused than the last. 
Planorbis marginatus —(the Flattened Coil 
Shell) (PL XL, fig. 128).—The shell is of a 
brownish colour, with close irregular striations, 
concave above, and flat or slightly convex below; 
whorls, six; the diameter of the last whorl is 
equal to about one-fourth of the whole, and is 
rounded above, and strongly keeled below. In 
the young, the P. rhombaeus of Turton, the shell 
is more convex above, and with a deep ^ X^ 
concavity beneath, and a blunt keel. 
Monstrosities of this species occur with 
the volutions elevated into a spiral cone^ 
as in fig. 27; or with the first few 
whorls coiled in the normal way, the others 
spiral; or with open whorls. 
This snail is prolific, and lays from eight to ten 
capsules, each containing from six to twenty eggs. 
