36 LAND AND FRESH-WATER MOLLUSKS. 
The same locality supplied, at the same time, 
a species new to Britain,— 
Cyctas ovatis—(the Oval Cyclas) (Pl. II1., 
fic. 10)—is a species intermediate in size and 
form between C. rivicola and C. cornea. Aver- 
age-sized specimens measure half an inch long, 
a quarter of an inch thick, and three-eighths 
wide. From C. rivicola it is distinguished by 
its oblong shape, its pale-drab colour, and 
fainter concentric striz, 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. 
II1., 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. 
