38 THE AMERICAN MUSEUM JOURNAL 



The Dromathenuiu and Microconodon casts and models rep- 

 resent two little jaws which are the oldest mammals known. 

 They were found in the Triassic coal-beds of North Carolina, 

 and have a type of teeth intermediate between the primitive 

 three-cusped form of mammals and the simple one-cusped tooth 

 of Reptiles. 



THE ORIGIN OF MAMMALS. 



The several groups of small and mostly primitive mammals 

 which we have gone over in this guide-leaflet furnish a great 

 deal of evidence, direct and indirect, as to the characters and 

 appearance of the ancestral group or groups from which all 

 mammals are supposed to be descended. 



The most ancient mammals, the Multituberculates and Meso- 

 zoic Trituberculates of the Age of Reptiles, are known only from 

 rare and very fragmentary remains and many more or less con- 

 tradictory hypotheses have been advocated as to the relations 

 of these little known groups to the Tertiary mammals which 

 succeeded them and to the lower vertebrates (reptiles and am- 

 phibians) which preceded them in geological history. The con- 

 sensus of present opinion is that mammals in general evolved 

 from an unknown stock of reptiles most nearly related to the 

 Theriodontia ' which flourished at the end of the Palaeozoic era ; 

 that they soon split into two branches, one of which (Mono- 

 tremes) survives without very great change in the egg-laying 

 mammals of Australia. The other more progressive branch 

 again split, one division giving rise to the Marsupials, the other 

 to the Placentals, the latter including all the remaining groups 

 of mammals. The Multituberculates represent a side branch, 

 but whether of Marsupials or of Monotremes is uncertain. The 

 Trituberculates of the Mesozoic era were more nearly in the 

 direct line; some appear to be in the Marsupial branch, others 

 more doubtfully in the Placental branch. But all these con- 

 clusions are largely hypothetical. 



When we come to the Tertiary mammals, we can speak much 

 more certainly, as these are far more completely known. All the 

 modem races of mammals, as we trace them back towards their 



• Exhibited in the Hall of Fossil Reptiles, south side. 



