50 GEOLOGY. 



mingled with the Mesozoic forms; in other words, there was a gra- 

 dation of the Paleozoic forms into the Mesozoic. 



The transition fauna appears to have been richer in this region 

 than that in the Mediterranean basin. In the Yakutic stage, a division 

 of the early Trias, there are now known to have been two hundred 

 and twelve species of cephalopods in the Indian province, against twenty- 

 five known at the corresponding stage in the Mediterranean province 

 (J. Perrin Smith), which is the more notable since the latter has been 

 much more thoroughly studied. Because of the superior richness, as 

 well as the close continuity of the life of the Himalayan province, it 

 is entitled to be styled the cradle of the Mesozoic fauna par excellence. 

 More strictly, however, it was the cradle of a leading provincial fauna 

 of the early Mesozoic only. The Mediterranean province soon developed 

 a vigorous rival fauna which deployed so strongly in the later Trias, 

 that it is regarded as the more representative fauna. 



Concerning the early stages of the Siberian fauna very little is 

 known; but its peculiarities, as they were better revealed in a later 

 stage of the early Trias, leave little doubt of its independence of origin. 

 Of other transition provinces still less is known. It is significant, 

 however, that an important group of ammonites (Tropitidce) appeared 

 in the Eurasian provinces suddenly and in great force, toward the 

 middle of the Triassic. As these ammonites had no immediate ances- 

 try within these regions, it is inferred that they were immigrants from 

 some other originating tract, and this tract will doubtless be discovered 

 in time as the study of other regions progresses. 



In a minor way, the general coast tract of all the continents, though 

 narrow, was doubtless the originating tract of some species, and per- 

 haps of minor faunas. 



It is scarcely necessary to remark that the pelagic and abysmal 

 life of the main ocean is not embraced in this review, and is practically 

 unknown. 



General nature of the faunal change. — In nearly all the Paleozoic 

 faunas, the brachiopods were a leading element, while the trilobites, 

 crinoids, corals, and orthoceratites, each in turn, gave distinctive 

 character to the successive faunas. In the Mesozoic era the ammon- 

 ites took the first place, followed b}' the pelecypqds and the gastropods. 

 The ammonites (Fig. 344) were peculiarly fitted for distinguishing 

 successive horizons, not only because they were free forms, measur- 



