CYCLAD^. 
251 
These shells are distinguished from the marine 
VenevidcB by the shell being covered with a hard^ 
olive, horny periostracum. 
Fig. 60. 
1 — 3. Cyclas cornea. — a, Lower, 6, Upper siphon ; Foot. 
As Mr. Jenyns justly observes in his excellent 
Monograph of the British species of this family, 
which has here been followed; all the species breed 
readily in confinement, during the spring and sum- 
mer months. They are probably oviparous ; and 
the young appear to remain for a certain period 
within the folds of the branchiae previously to their 
exclusion, since many may be found of different 
sizes within the parent at one and the same time. 
They have the faculty of producing long before they 
are arrived at their full growth ; and even some in- 
dividuals, which are themselves so immature as to 
possess hardly any of the distinguishing characters of 
the species, frequently contain young of a sufiicient 
size to be seen from without through the transparent 
valves. 
When kept alive, they readily and frequently 
ascend the sides of the vessel, and glide along the 
surface of the water, with their foot extended on it, 
and the shell immersed and in an inverted position. 
In this manner, like the Limncei and other Gaste- 
