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, Exocjym sub-orhictdata f= columba, Desh.). Sometimes the beaks of 

 the attached valve become quite indistinct, as in Kvoyyra Coiiloiiii, d'Orb., or in 

 the South Indian Ex. fausta. 



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

 E. Boussiugaultu, 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 Amjihidonta (1829) is applicable. 



The Exogyrce are always uusymmetrical, being attached with the right valve 

 laterally at the umbo, while the same valve of the GryphcecB 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. 



Coquaiid lias very recently published a " Monograpliie 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 

 repeat 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 whicli I have already indicated. 



As regards the geographical distribution of the cretaceous OSTREID^, Coquand gives 

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



Lower- Middle- Upper- cretaceous. 



European species (peculiar) 



African „ „ 



Asiatic „ „ 



American „ „ 



Number of species common to Europe and Africa ... 

 Ditto ditto ,, „ Asia 



Ditto ditto Africa „ „ 



Ditto ditto Europe, Africa, and Asia 



Ditto ditto „ and America 



Ditto ditto Asia (India) and America 



Ditto ditto Europe, Africa, Asia, and America 



Total of species occurring in the old Continent ... 220 ) ' 



Ditto ditto new ditto ... 53 j 



Ditto common to both 



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^ 

 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. 



