CYCLADJS. 251 



These shells are distinguished from the marine 

 Veneridcs by the shell being covered with a hard, 

 olive, horny periostracum. 



Fig. 60. 



1 3. Cyclas cornea. a, Lower, 6, Upper siphon ; c, 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 sufficient 

 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- 



