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SUPPLEMENT TO THE BRITISH 



91. Rhynchonella Cantabrigensis, Dav. Sup., PI. VIII, figs. 30, 30«, 30<5. 



Spec. Char. Obscurely deltoid, about as broad as long, greatest breadth towards 

 the posterior portion of the shell, tapering posteriorly to the extremity of the beak. 

 Dorsal valve convex or gibbous, divided into three portions, of which the central one 

 forms a convex mesial fold. Ventral valve very slightly convex or flattened, of small 

 depth, and divided by a broad mesial sinus, which deepens as it nears the front. 

 Front deeply sinuated ; beak slightly produced ; foramen small, situated under its 

 gently incurved angular extremity, and bordered by a narrow deltidium. A flattened 

 space exists between the beak-ridges and hinge-line. Surface of each valve marked by 

 from twenty-four to twenty-eight angular ribs, which are, in some specimens, stronger or 

 larger on the lateral portions of the valves than on the fold and sinus. 

 Length 10,width 10^, depth 8 lines. 



05s. Mr. Walker considers that the species under description is nearest to Rh. 

 parvirostris, but more elongated and globose. These and other differences become 

 very apparent when one compares with it true specimens of Rh. parvirostris from 

 the Isle of Wight. In Rh. parvirostris the greatest breadth of the shell is at or close to 

 the hinge- line, while the beak and its lateral slopes form a broad obtuse angle with their 

 outer extremities turned upwards. In Rh. Cantabrigensis the reverse is observable ; the 

 beak-slopes merging gradually into the general lateral curve ; and while one species is 

 decidedly transverse, the other is about as wide as long. 



Position and Locality. Exceedingly abundant in the Lower Greensand at Upware : 

 some specimens agreeing with these have been found in deposits of a similar age at 

 Godalming and the Isle of Wight. 



92. Rhynchonella Walkeri, Dav. Sup., PI. VIII, figs. 33, 34. 



Spec. Char. Almost circular or about as wide as long. Valves moderately convex ; 

 dorsal valve divided into three almost equal portions, of which the central one forms the 

 mesial fold ; ventral valve rather less convex than the opposite one, sinus wide, of 

 moderate depth ; beak short, incurved, leaving between its ridges and the hinge-line a 

 flattened space ; foramen minute, margined and separated from the hinge-line by a 

 narrow deltidium. Surface of each valve marked with from eleven to fourteen strong 

 angular ribs, of which from three to four compose the fold, two to three the sinus. 

 Length 5, width 9^, depth 5 or 6 lines. 



Obs. This species is easily distinguished from any of the other forms that occur in 

 our British Cretaceous rocks by its few and large angular ribs. It was discovered by 



