212 LAND AND FRESH-WATER MOLLUSKS. 
: Shell very flat beneath, prone keeled ... P. vortea 
* ( Shell subcarinated ee .. L. sprrorbis 
8 Shell very small, imbricated se .. P. nautileus 
" ( Shell large vo, Cee 
9 Keel on the middle of the iy ... P. carimatus 
" ( Keel on the lower side... ...  ... P. marginatus 
Bluntly keeled in the middle wi vee) ae 
Septa in the last whorl... ... ... ... 2. lineatus 
| PLanorBis cornEus—(the Horny Coil 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 cone lines 
of growth. ‘The whorls are five in number, with 
a broad shallow umbilicus on the under side, the 
upper surface a httle 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 
