Prof. E. Forbes on Fossil Invertebrata from Southern India. 1G5 



and the French " Neocomian" species, whose names I have adopted, I can draw 

 no specific distinctions ; and two Nautili which appear to be identical with the 

 Nautilus l(svigatus and Nautilus clementinus of M. D'Orbigny, species from creta- 

 ceous beds in France. In the Verdachellum collection we have Pecten quinque- 

 costatus again, quite undistinguishable from Blackdown examples, Pecten ohliquus, 

 precisely the same as specimens from the Isle of Wight, and fragments of a Pecten 

 apparently identical with our Pecten orbicularis. Also Trigonia aliformis and Car- 

 dium Hillanum, very characteristic cretaceous forms, and an Oyster, in all proba- 

 biUty one of our greensand species. In the Trinchinopoly collection are Cardium 

 Hillanum, apparently common, and a Pecten which I cannot separate from the 

 greensand Pecten virgatus. 



On the other hand, however similar at first glance some of the shells may seem 

 to tertiary species, on close inspection they have, every one, proved to be distinct. 

 So far then as positive identifications go, the plain inference respecting the age of 

 the beds at the three localities is, that they are Cretaceous. 



However clear this conclusion may appear, it cannot be unhesitatingly received, 

 even upon such evidence, in the case of beds so far distant and under such very 

 different circumstances of climate, &c., from any cretaceous beds with which we 

 are sufficiently acquainted. It might be that in the region in which these strata 

 were formed, certain species had lived on to a later epoch than they had in less- 

 favoured localities ; or, having commenced their existence at one point in space at 

 an early epoch, they might in the course of time become so distributed, that at 

 length, in consequence of changes in the distribution of land and water, they 

 would survive at a later period only in some sea far removed from their birthplace. 

 These possibilities must always be taken into account in all comparisons of geolo- 

 gical formations far apart. Except in the case of species known to have an 

 extended vertical as well as horizontal range and yet to be distinctly limited to an 

 epoch, it is dangerous to draw conclusions as to the synchronism of beds under 

 such circumstances. In this case, however, the bivalve mollusca named as iden- 

 tical with European species are all widely distributed forms, and yet everywhere 

 distinctly cretaceous. 



But the inference of the cretaceous date of the Southern Indian beds is borne 

 out by, to my mind, higher considerations than such identifications of species. 

 Among the facts of a general character made known to us by palseontological 

 research, there is, perhaps, none more interesting than the restriction of the minor 

 groups in large genera, capable of considerable and definite variations of form, to 

 limited portions, so to speak, of time, and the consequent indication afforded us of 

 the probable age of the strata in which they occur, in the absence of familiar forms. 

 This is also true of the arrangement in time of genera in certain families. Ammo- 



VOL. VII. SECOND SERIES. Z 



