450 MESOZOIC ERA— AGE OF REPTILES. 



reptiles reaching upward and forward toward mammals. In Jurassic 

 and especially Triassic mammals we see this class reaching downward 

 and backward toward reptiles. But the point of union has not yet 

 been found. The lowest and most reptilian of mammals, the egg-lay- 

 ing monotremes, have not yet with certainty been found fossil; but 

 the calcified teeth recently found in the embryo of the Ornithorhynchus 

 so strongly resemble the teeth of one family of Mesozoic mammals — 

 viz., the Multituberculata (Fig. 840, p. 500) — that it is now believed 

 that these were, indeed, monotremes. It is probable, therefore, that 

 the point of union between the classes reptiles and mammals will be 

 found, not, indeed, in monotremes proper (for these are already special- 

 ized), but in a generalized type connecting monotremes and marsupials. 

 The time of origin was probably the Lower Trias or Upper Permian. 

 The earliest mammals, such as the Microlestes and the Dromatherium 

 from the American Trias, were not far removed from such a generalized 

 type. The order of evolution has been expressed by Huxley thus : 1. 

 Hypotheria (below mammals) ; 2. Prototheria (first or lowest mam- 

 mals) ; 45. Metatheria (transition mammals) ; and, 4. Eutheria (perfect 

 or true mammals). The first is represented by the Theromorphs of 

 Permian .and Trias, the second by a hypothetical generalized type con- 

 necting monotremes and marsupials of the Triassic, the third by insec- 

 tivorous marsupials of the Jurassic, and the fourth by the true placen- 

 tal of the Tertiary. Further, it is probable, as suggested by Osborn,* 

 that " the Prototheria of Triassic separated very early into two branches 

 of Metatheria — one more like the marsupial, the other like insectivora. 

 From the latter came the Eutheria, which again differentiated into 

 many specialized orders. 



Mammals, then, existed in considerable numbers in the Jurassic. 

 These, however, were not able to contend with the great Dinosaurs. 

 It was still the age of reptiles. This class not only predominated, but 

 impressed their character on all higher classes. The birds and the 

 mammals were still reptilian. From the reptilian stem the bird and 

 mammal branches were not yet so completely separated that connect- 

 ing links were obliterated. 



Section 3.— Juka-Tkias in Amekica. 



We have already explained that these two periods are not well sepa- 

 rated in America. This is partly on account of the poverty of fossils, 

 and partly on account of the continuity of conditions throughout. It 

 seems best, therefore, in the present state of knowledge to treat them to- 

 gether as one period. Doubtless they will be better separated hereafter. 



* Mesozoic Mammalia, p. 261, Transactions of the Academy of Sciences, Philadelphia, 

 vol. ix, No. 2, 1888. 



