PEHXA. 



91 



Ventral margin curved reo•ular]3^ Dorsal part of anterior margin concave. 

 Valves moderately convex near the anterior margin, but flattened clscwliere. 

 Dorsal half or more of the anterior marginal parts concave and depressed. Umbones 

 sharp, close together, projecting beyond the rest of the anterior margin of the 

 valves. 



Surface with growtli-lamella) at more or less regular intervals. 



Mensiiremenf.'^ ; 



(1) (2) (3) 



105 . 91 . 75 mm. 



1 1<» . i(i<; . kh „ 



(1—3) Pernn-hcd, AtlievfieW. 



Length of liinge 

 Height of valve 



V- 



Fill. 17. — Perna Ricordeana, (VOrbiijny- Lower Greensand (Pcrno-bed), Atlioifiold. Seilfovick Museum. 

 Eight valve and anterior view of both valves, x J. 



Affijiities. — Examples of this species from the Lower Greensand of the Isle of 

 Wight were regarded by Pictet and Campiche as distinct from P. li'icordcaim and 

 were described as Perna Fittoni. According to those writers the former is 

 distinguished from the latter chiefly by the wide separation of the umbones ; this 

 separation, however, seems to me to be due to the thickening of the valves in old 

 age, such as may be seen not infrequently in GerviUia and other allied forms. 

 The postero-dorsal angle of the valve, according to d'Orbigny's figure, appears to 

 be rather smaller in P. EkonJmna than in P. Fittoni, but the difference is not 

 great, and moreover, the outline of the shell and the size of this angle vary in 

 different specimens of P. Fittoni. /'. German i, Pictet and Campiche, is very 

 closely allied to P. Ricordeana. 



