106 MOLLUSCA FROM THE CRAG. 
Cycuias.* Bruguiere, 1792. 
Spumrium. Scopoli, 1777. 
Nux. Humph., 1797. 
Cornea. Megerle, 1811. 
CornetocycLas. Ferus., 1818. 
Generic Character. Shell equivalve, subequilateral, more or less tumid or inflated, 
thin and closed, sometimes semipellucid, smooth or slightly marked by lines of 
growth, and in the recent state covered with an epidermis. Hinge furnished with 
one or two cardinal teeth, and distant lateral teeth on each side. Impressions of the 
adductor muscles indistinct. Palleal impression with a small sinus. Ligament 
external, slender. 
Animal suborbicular, its mantle open in front, with plain or simple margins ; 
siphon produced and divided at the extremity into two distinct tubes, the edges plain 
or without fringes: foot large, compressed, extensile, and more or less pointed. 
Priority of name most properly belongs to Scopoli, but the small and corneous 
shells here included are so universally known by the above designation, that I do 
not feel disposed to make the alteration, more particularly as Spherium has been 
since adopted in another department of Natural History as a Generic Term. Animals 
now determined to belong to three distinct Genera were included by Bruguitre, as 
well as by Lamarck, under the name of Cyclas, and the latter author subsequently 
proposed to sever from them the thicker and more ponderous species, and unite them 
into a genus by themselves, under the name of Cyrena. 
The shells constituting this genus are for the most part very thin, and of a 
corneous or semitransparent texture in the living state; their little mhabitants are 
possessed of considerable powers of locomotion, and move about in the water with 
facility by means of their large and flexible foot; they frequent pools, ditches, lakes, 
and sluggish streams, and when still, are generally found buried in the sand or muddy 
bottom of the water. They are viviparous or rather ovoviviparous, and the young are 
not only perfectly formed before exclusion, but are sometimes of considerable magni- 
tude, occupying a large portion of the parent shell to the manifest inconvenience of 
the mother. 
They are purely Fresh-water Molluscs, and the Formations in which they are 
found fossil, are either of Fresh-water origin or of Estuaries in close proximity into 
which they have been washed. Species have been figured and described as belonging 
to this genus from the Wealden Formation ; an undoubted Cyc/as was found by myself 
in the Fresh-water Deposit at Hordwell, belonging to the Older Tertiaries. 
* Etym. ku«dds, circular. 
