278 



CONCKIFERA, 



1 . 3. Cyclas cornea. a lower, b upper siphon ; c the foot. 

 4. 5. Pisidium amnicum. d the single siphon j c the foot. 



These shells are distinguished from the marine 

 Veneridae by the shell being covered with a hard 

 olive horny periostraca. 



As Mr. Jenyns justly observes, in his excellent 

 Monograph of the British species of this family, 

 which has here been followed, that all the species 

 breed readily in confinement, during the spring and 

 summer months. They are probably ovoviparous, 

 and the young appear to remain for a certain pe- 

 riod within the folds of the branchiae previous to 

 their exclusion, since many may be found of dif- 

 ferent 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 individuals, which are themselves so immature 

 as to possess hardly any of the distinguishing charac- 

 ters 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 man- 



