36 
LAND AND FRESH-WATER MOLLUSKS. 
The same locality supplied, at the same time, 
a species new to Britain,— 
Cyclas oyalts —(the Oval Oyclas ) (PL III., 
fig. 10)—is a species intermediate in size and 
form between 0 . rivicola and 0 . cornea . Aver¬ 
age-sized specimens measure half an inch long, 
a quarter of an inch thick, and three-eighths 
wide. From 0 . rivicola it is distinguished by 
its oblong shape, its pale-drab colour, and 
fainter concentric striae, and more markedly by 
its straight hinge-line. It has since been found 
in the Surrey Canal, at Exmouth, and in Lan¬ 
cashire. It occurs in marshes in the North 
of France. 
Cyclas lacustris —(the Capped Cyclas ) (PI. 
III., fig. 12).—The shell of this species contrasts 
strongly with those of the other Cyclads, in its 
sub-rhombic form, much compressed, thin, of a 
yellowish-white colour; in the prominent um- 
bones, which are narrow, and projecting like 
little caps, from which latter character it has also 
received the specific name calyculata . 
The shell is small, delicate, and shining, four 
lines long, three wide, and one and a half thick. 
From the extreme thinness and semitranspa¬ 
rency of the shell, the young, the lamellated 
gills, and the pulsating heart may be easily seen 
within. 
C. lacustris has much the same habits as C. 
