WATER SNAILS. pales 
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. 
PLANORBIS SPIRORBIS—(the Round-edged Coi 
Shell) (Pl. XI., fig. 129)—The shell of this 
species 1s very closely allied to that of P. vortea, 
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. 
Jt is more widely diffused than the last. 
PLANORBIS MARGINATUS—(the Flattened Coil 
Shell) (Pl. X1., 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. rhombeus of Turton, the shell 
is more convex above, and with a deep AY 
concavity beneath, and a blunt keel. -—~\ 
Monstrosities of this species occur with es =e 
the volutions elevated into a spiral cone; Ss 
as in fig. 27; or with the first few Fis. 27. 
whorls coiled in the normal way, the others 
spiral; or with open whorls. 
This snailis prolific, and lays from eight to ten 
capsules, each containing from six to twenty eggs. 
