OF SOUTHERN INDIA. 455 



genus. The one typical form has the left valve thin, quite flat, or slightly convex ; 

 the inner mantle edges are mostly smooth, the right valve usually of a roundly 

 ovate, semi-globular or tumid form ; a well known cretaceous form of this type is, 

 for instance, Exogyra sub-orUculata f= columha, Desh.). Sometimes the beaks of 

 the attached valve become quite indistinct, as in Bxogyra Coulonii, d'Orb., or in 

 the South Indian Ex, fausta, 



3^. The other section which may perhaps be best typified by the cretaceous 

 E. BoussingaulUi, d'Orb., has a sigmoid shape ; the left or smaller valve is often 

 considerably raised at the margins, flattened or sometimes even concave about the 

 middle. To this section Fischer's name AmpUdonta (1829) is applicable. 



The ExogyrcB are always unsymmetrical, being attached with the right valve 

 laterally at the umbo, while the same valve of the OryphcBcE is sometimes appa- 

 rently quite symmetrical, and if there is a place of attachment it is almost or nearly 

 central. The character of the ligamental groove in the two valves forms a good 

 distinction between the two genera. 



List of cretaceous species. 



Coquand has very recently pablished a '^ Monographie du genre Ostrea'' embodying all 

 the species which have been described from authentic cretaceous rocks up to 1869. The additions 

 and alterations of specific names will be extremely few. It would hardly appear necessary to 

 3'epeat here all the names, were it not for the purpose of completing the list of all cretaceous 

 Pelecypoda, known up to date. I shall enumerate the species according to their geological posi- 

 tion in upper, middle, and lower cretaceous rocks, and shall at the same time arrange them according 

 to the three generic distinctions which I have already indicated. 



As regards the geographical distribution of the cretaceous OSTREIDJE , Coquand gives 

 detailed data which, with slight alterations, can be put as follows : — 



Lower- Middle- Upper- cretaceous. 



European species 



(peculiar) 



... 



26 



38 73 



African 



i> 



J5 





... 



5 



« 



5 14 



Asiatic 



jj 



)J 





... ... •• . ... 



1# 



6 7 



American 



jj 



JJ 





... 



4 



51 ? 



Number of 



species common 



to Europe and Africa ... 



.. . 





23 



Ditto 





ditto 





„ Asia 







17 



Ditto 





ditto 





Africa „ „ 



, , 





2 



Ditto 





ditto 





Europe, Africa, and Asia 



.. 





10 



Ditto 





ditto 





„ and America 







2 



Ditto 





ditto 





Asia (India) and America 







1 



Ditto 





ditto 





Europe, Africa, Asia, and America 



. , 





6 









Total of species occurring in the old Continei 



at 





220 











Ditto ditto new ditto 







53 











Ditto common to both 



,, 





9 



}- 



273 



It deserves to be noticed that the geological positions in which those species common to 

 Europe and India occur are strikingly identical in the two countries, showing that the Ostreid^e 

 can be considered as good characteristic fossils in tracing out the relative age of different rocks. 



* The boundaries in European and Asiatic Russia are unsatisfactorily recorded. 



