416 THE MEDALS OF CREATION. Crap. XI. 
fossils afforded in the determination of the fluviatile origin 
of thg Wealden, for our Excursion to Tilgate Forest. 
Cycias. Wond. p. 404. Ly. p. 28.— Another genus of 
fresh-water bivalves is termed Cyclas, of which there are 
ten species in the Wealden formation : and, with the excep- 
* tion of four or five recent forms, which occur in the tertiary 
fresh-water strata, none others have been found in England.* 
. 
Lien. 182. CycLas AND MELANopsIS. Wealden; Sussex. 
The shells of the genus Cyclas are oval, transverse, equivalved 
bivalves, with the hinge-teeth very small: the substance of 
the shell is thin and fragile ; the figures in Wond. and Ly. 
accurately represent the appearance of the fossil Cyclades 
of the Wealden, and tertiary strata. Entire layers of two 
or three species of these shells occur in the argillaceous 
deposits of the Wealden, generally in a friable state, but 
from among the masses of crushed shells, perfect specimens 
may be obtained, and sometimes with the remains of the 
epidermis and ligament. The hard stone, termed calciferous 
* Cyrena, is a genus so nearly related to Cyclas, that it is difficult 
to distinguish them, and it will be convenient to retain only the 
former name. 
