508 CRETACEOrS PELECYPODA 



General results derived prom the examination oe the South Indian 



CRETACEOUS PeLECYPODA. 



In a similar way, as I have done with the Cephalopoda and the Gastropoda,"^ 

 I shall again briefly summarize the general palseontological and geological facts 

 which have been derived from the study of the Pelecypoda. The richness of this 

 class of Mollusca is equally great as that of the other classes already referred to, and 

 among the various genera there are several which were up to date very imperfectly, 

 or not at all, known to occur in cretaceous deposits. Eor this reason the study of 

 the South Indian cretaceous Pelecypoda has been particularly interesting, audit 

 is to be hoped that it will throw some light upon the correct generic determination 

 of the cretaceous Pelecypoda in general. 



I have described in the present volume 243 species, of which only very few 

 are not characterized with sufficient detail, the materials for that purpose being as 

 yet imperfect. 



It is a noteworthy fact that the total (243) number of species of Pelecypoda 

 is very nearly the same as that of the Gastropoda. The cretaceous (particularly 

 the middle and upper) fauna holds in this respect an intermediate position. Eor 

 during the kainozoic period the Gastropoda preponderate over the Pelecypoda, 

 and the difi'erence is still more marked within the recent epoch. On the contrary, 

 during the Jurassic period the Gastropoda are smaller in numbers when compared 

 with the Pelecypoda, and the same is also the case in the preceding periods. 



The 243 species are distributed in 9 orders and 31 families (out of 10 orders 

 and 46 families into which I have respectively classed all known fossil and 

 recent Pelecypoda). Of genera (including sub-genera — these not being separately 

 counted) there are 85 represented out of the much larger number which I have 

 noticed either recent or fossil (see Preface p. ix and seq.). 



In order to indicate the zoological value of the examination of the South 

 Indian cretaceous Pelecypoda, I shall briefly mention the number of species 

 in each order : Froladacba with 11 ; Myacjsa with 25 ; Tellinacea with 12 ; 

 Veneracea with 41 ; Chamacea with 4 ; Lucinacea with 19 ; Arcacea with 38 ; 

 Mytilacea with 29 ; Ostreacea with 65, The only order of which no repre- 

 sentative was found is that of the true fresh-water Pelecypoda, the Unionacea. 

 The Ostreacea^ being the only true Monomyarian Pelecypoda, and of the lowest 

 organization, are, as may be expected, most numerous. Of the other orders I 

 would only allude to the Chamacea which are comparatively very little represented, 

 although they include the almost exclusively cretaceous family Sippuritidje:, This 

 want seems to be due to the absence, or non-exposure, of coral-reefs and littoral 

 cliffs. 



In a geological point of view the results are equally interesting. The examin- 

 ation has, again, in general proved to be in favour of the tri-division into which 

 the South Indian cretaceous deposits of the Trichinopoly district have been divided 



* Palseontologia Indica, Vols. I and II, of the fauna of the South Indian cretaceous rocks. 



