LAMELLIBRANCHIATA. 125 



and pointed, directed forwards and incurved. Anterior side 

 rounded ; posterior side slightly produced, its margin rounded ; 

 antero-cardinal region hollowed, but with no defined lumule. Pos- 

 tero-cardinal area well-defined by a strong keel, which arches 

 backwards from beneath the umbo to the postero-lateral margin, 

 so as to mark off the posterior lunette. Behind this keel comes a 

 second oblique ridge, which is however very feebly developed ; it 

 produces a slight angle in the pallial border of the shell, which is 

 elsewhere simply curved. 



Affinities and differences. Cyprina cuneata, C. Saussuri 

 (Brong.) and C. obtusa all belong to one group of Cyprina charac- 

 teristic of the cretaceous rocks, and distinguished by their wedge- 

 shape, produced posterior side, and the presence of two posterior 

 oblique ridges. They are closely allied to one another, and may, 

 I believe, prove to be only varieties of one species. But materials 

 are not yet collected to prove this, though the idea is supported 

 by some Atherfield specimens. 



As compared with Cyprina cuneata, Sby., the umbo is less 

 prominent and more rounded, all the angles less sharp and the 

 posterior side less produced, so that the pallial border is scarcely, 

 if at all, sinuated. The whole shell is less angular and more 

 tumid, hence the name applied to it. In many of these differences 

 our shell approaches C. Saussuri (Brong.), but that shell is of a 

 different shape — not so trigonal — also its umbo seems to be more 

 prominent, and the posterior end is more produced. 



In general shape it is near to C. Bernensis Leym. (l'Aube, pi. 

 v., f. 5). 



Localities. Upware, Folkestone, base of Folkestone Beds (Mi- 

 Meyer's collection). 



Venus Vectensis, Forbes. 



Venus Vectensis, Forbes, 1845. Quart. Journ. Geol. Soc, Vol. I., 

 p. 240, pi. II., f. 4. 



So far as I know only one specimen has occurred, and that a 

 very indifferent one (Woodw. Mus.) 



A specimen in the University collection from Atherfield shews 

 the hinge teeth in the right valve. There are three cardinal teeth 



