222 Evolution and Distribution of Fishes 



"The thickness of the formation is, on the whole, rather 

 uniform, averaging perhaps 200 or 300 feet, though greater 

 thicknesses are known. To the South (Texas) the Dakota 

 formation rests on the Comanchean system unconformably. 

 Farther north it is often in apparent conformity with the 

 Comanchean, though it often, as in the Wasatch and Uinta 

 Mountains, rests on older formations." (op.cit.pp. 145-46). 



While the sea seems to have made great inroads on 

 the land, during most of the Upper Cretaceous, and furnish- 

 ed wide opportunity for migration into it, and evolution in 

 it, of races of elasmobranch and teleost fishes, a gradual 

 readjustment took place, with corresponding reformation 

 in late Cretaceous of lakes that rivalled those of the earlier 

 or Lower Cretaceous. One of the most extensive of these 

 again was that over the bed of which the Laramie strata 

 were laid down in Western America. These strata vary 

 in thickness from 1000 to 5000 feet, and from the conjoint 

 researches of L. F. Ward, of White, of Cope, of Dawson 

 and of Tyrrell, the lake or lakes became the centre for pres- 

 ervation of a varied flora, of a mammalian fauna that was 

 mainly marsupial, of a remarkable avian and reptilian 

 fauna, of a fish fauna described mainly by Cope, and of a 

 rich freshwater molluscan fauna. 



In India and elsewhere, deposits of like extent and 

 history can be traced. 



When such facts as the above are correlated, it will be 

 seen that the Cretaceous formation is by no means the pre- 

 dominating marine one that geologists and palaeontologists 

 have often supposed. One great reason for the latter 

 view has been that multitudes of the marine organisms 

 found as fossils readily lent themselves to fossilization amid 

 the deposits in which they are embedded. Thus the fora- 

 minifera, the corals, the crinoids, echinoids and starfishes, 

 the crustaceans, and the molluscs, were — as is usually true of 

 marine invertebrate life — prodigiously abundant, and be- 

 came quickly preserved in calcareous rocks. Readily lend- 

 ing themselves also to the work of the systematist, they have 

 been elaborately described and figured, often at the ex- 

 pense of higher types that require more discrimination in 

 study. 



