216 LAND AND FRESH-WATER MOLLUSKS. 
It can fix its shell without any apparent means of 
attachment by its side to the flat surface of the 
aquarium glass, where it may be often found 
left high and dry by the loss of water in the 
glass by evaporation. The flattened coil shell, 
so frequent in the ponds and ditches of the south 
and midland counties of England, becomes rare 
northwards, and is unknown in Scotland; it 
occurs throughout Ireland. 
A well-known synonym ee this species is P. 
complanatus. 
PLANORBIS CARINATUS—(the Keeled Coil Shell) 
(Pl. XI., fig. 1380)—The shell differs from that 
of the et in its broader and more depressed 
whorls, and the keel being situated on the middle 
line of the whorl instead of below. It is of much 
rarer occurrence, and never plentiful. It is 
found somewhat generally in the eastern coun- 
ties; it is doubtfully Scottish, but occurs in 
Treland. 
PLanorRBIS ALRUS—(the White Coil Shell) (Pl 
XI., fig. 125).—The specific name albus is ap- 
plied to this species from the greyish-white 
colour of the animal and shell. The head of the 
snail is thick and oblong in front, the tentacles 
are long, slender, of a pale grey or white, with a 
central brown line, dilated and transparent; at 
the base on the inner side of each tentacle is a 
small oval black eye. The foot is narrow, the 
