448 



MESOZOIC ERA— AGE OF REPTILES. 



consolidation was not yet complete. Fig. 705 represents wings of these 

 four kinds — the dotted lines run through corresponding parts, and 

 show the identity of structure. 



Origin of Birds. — There can be no doubt, then, that Birds came 

 from Reptiles. Further, it is most probable that they came from 

 Dinosaurs. It is true that Dinosaurs are the largest of reptiles, while 

 birds, with some exceptions, are comparatively small animals ; but Marsh 

 has shown that some American Dinosaurs were very small. The time 

 of their origin, or separation from the reptilian stem, is still doubtful, 

 but the wonderfully reptilian character of Archgeopter}^x shows that it 

 can not be far from the point of origin. It was probably in the Lower 

 Jurassic or Upper Triassic. 



Mammals. — We have already seen (p. 421) that the first appearance 

 of this class was in uppermost Trias ; but as these were few in num- 

 ber, and very near the confines of Jurassic, we regarded them as an- 

 ticipations, and put off their discussion. In the Jurassic this anticipa- 

 tion was fully realized. In the same formation (Upper Jurassic) in 

 which we found the old forest-ground (p. 425) have been found also 

 fourteen genera of small mammals. To this may be added five genera 



Fig. 707. 



Fig. 706. 



€8b3jfe> 



<** e a*aacwS<£e4 



Fig. 708. 



Fig. 709. Fig. 710. 



hitherinm Prevostii. 707. Phascolotherium. 708, 



Figs 706-710.— Jurassic Mammals: 706. Amphitherinm Prevostii. 70 

 Amphitherium. 709. Triconodon. 710. Plagiaul 



from a lower horizon (Stonefield states). If we add to these again four- 

 teen genera (twenty-five species), recently described by Marsh, from the 

 American Jurassic, we have at least thirty-three Jurassic genera known. 

 Besides these, at least five genera are found in the Upper Trias of all 

 countries, and sixteen genera (twenty-four species), recently (1889) 

 described by Marsh, from Upper Cretaceous (Laramie). We may say, 

 therefore, that there are now known at least fifty-four genera of Meso- 

 zoic mammals. The number of species is, of course, much greater. 



Affinities of Jurassic Mammals.— The Jurassic, and indeed the Meso- 

 zoic mammals, as contrasted with mammals of later times, may be 

 characterized in a general way as small insectivorous marsupials, or, 

 perhaps better, as a type connecting insectivores and marsupials, and 

 therefore lower and more generalized than either. They were, espe- 



