212 LAND AND ERESH-WATEE MOLLUSKS. 
vj \ Shell very flat beneath, strongly keeled ... P. vortex 
( Shell subcarinated . P. spirorbis 
( Shell very small, imbricated ridges ... P. nautileus 
\ Shell large . 9 
g ( Keel on the middle of the whorl. P. carinatus 
\ Keel on the lower side ... . P. marginatus 
jq ( Bluntly keeled in the middle . P. nitidus 
* ( Septa in the last whorl. P. lineatus 
Planorbis corneus— {the Horny Ooil Shell) 
(PL X., fig. 120).—"The shell of this species, the 
largest of the genus, is from half an inch to an 
inch in diameter; an unusually large specimen 
in my possession, from Tottenham, Middlesex, is 
an inch and a quarter across. The colour is a 
glossy, reddish-brown, sometimes white; the 
test thick and irregularly striated by curved lines 
of growth. The whorls are five in number, with 
a broad shallow umbilicus on the under side, the 
upper surface a little concave. The young shell 
has its surface of a velvety appearance, on 
account of the epidermis being clothed with a 
fine down. 
The body is nearly black; the tentacles are 
long and curved. The jaw consists of three 
pieces, but the lateral ones are very rudi¬ 
mentary. A peculiarity of this animal, alike 
possessed by all the coil-shells, is that of emit¬ 
ting, when irritated, a purple-coloured fluid, 
which is secreted by a gland at the sides of its 
neck. It is a very sluggish animal, and on warm 
