BIVALVIA. 103 
perfection. In all probability it was equally variable in the earlier periods of its 
existence, as it is at the present day. The few specimens that I possess present 
considerable differences in outward character, from which, therefore, it is not unfair 
to infer, that amongst a greater number, and from different localities, we should, as in 
the living shell, which varies under different external conditions, also have a great 
variety in the fossil state. 
The specimens from Stutton (generally in a decorticated condition) are very in- 
equilateral, the anterior side being particularly short in proportion to the other, and 
the shell is rather less in length comparatively, but it is almost impossilile to obtain a 
specimen at that locality without some slight degree of distortion, and its true characters 
are therefore difficult to determine; but there is no doubt it is the homogenitor of our 
old acquaintance, which in the living condition puts on sucha variety of shapes. The 
one from Clacton is more elongate, and corresponds with a variety abundant in the 
Thames above Maidenhead, in Berkshire. Mr. J. E. Gray (Manual of Land and 
Fresh-water Shells of the British Islands) considers the many different forms found in 
this country, and described under distinct specific names, to be only modifications of 
the same species, and the authors of the ‘ History of British Mollusca’ are of the same 
opinion; from what I have seen I fully acquiesce in their decision, and the fossil 
specimens that have come under my observation may be arranged in the same 
category. 
As the geographical range of this species extends over the greater part of Europe, 
we may naturally expect to find it in the fossil state. 
Cyrena, Lomarch. 1818. 
Venus (sp.). Chem. 
TELLINA (sp.). Gmelin. 
Cyctas (sp.). Lam., 1799, 1801. 
CorsicuLa. Megerle, 1811. 
Cyanocycias. Ferussac, 1818. 
GrLomna. Gray, 1844. 
Verona: Ids. .; 
Generic Character. Shell equivalve, roundly trigonal or subovate, generally 
tumid or inflated, more or less inequilateral, thick and strong. Hinge with three 
cardinal teeth, and a large extended lateral tooth on each side, serrated or striated in 
some species: in the recent state covered with an epidermis: ligament external: 
impression of the mantle without a sinus. 
The animals constituting this genus are in a recent state inhabitants of pure 
fresh-water, and are only known in tropical or sub-tropical rivers, where the climate is 
particularly mild. They are described by Malacologists as not differing essentially 
