CONCHIFERA. 



233 



mantle freely open in front, and anteriority ; posteriority 

 united to form a single siphon, composed of the united 

 anal and branchial tubes ; margins of its orifice, and of the 

 mantle simple; foot large, linguiform, and very exten- 

 sile.* — Several species ; also fossil. 



There are seven British species, all small, living in lakes, 

 ponds, ditches, and springs. Mr. Gray says that they 

 have the power of gliding 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 Gasteropods, they contrive to traverse the 

 vessel from side to side, as though they were crawling 

 along a solid plane, f 



Cyrenoida. De Joannis. (Cyrenella Desk.) — Shell 

 equivalve, sub-equilateral, ventricose, thin, covered with a 

 reddish brown epidermis, corroded at 

 the umbones, and with a slight 

 posterior fold; hinge thin, with 

 three diverging cardinal teeth in 

 each valve, and a very slight pos- 

 terior fold in the right valve; li- 

 gament not very tumid. % Animal 

 with a widely cleft, entire bordered 

 mantle, two siphons connected to- 

 gether half the length of the shell ; 

 foot cylindrical, blunt, rather large ; two striated gills ; 

 four lancet-shaped striated lip tentacles. 



A fresh water shell slightly differing from Cyclas and 

 Cyrena. 



Iphigenia. Schum. (Capsa Lam.) — Shell equivalve, 



* Forbes's British Moll. ' f Land and Freshwater Shells. 



% Sow., Conch. Manual. 



