EHE 
AMERICAN NATURALIST 
VoL. XXXII. May, 1898. No. 377. 
THE ORIGIN OF THE MAMMALIA 
HENRY FAIRFIELD OSBORN. 
THE most important problems in vertebrate morphology at 
the present time are the connections which once existed between 
the great vertebrate classes. As regards the three lower classes, 
the present state of opinion is as follows: The Amphibia are 
derived by Pollard, Cope, Dollo, and Baur from the ancient 
crossopterygian fishes, an order represented by the modern 
Polypterus and Calamoicthys, the Dipnoi being regarded as a 
parallel rather than an ancestral line. The Reptilia, as repre- 
sented by their most primitive order with solid-roofed skulls 
(Cotylosauria, Cope, or Pareiasauria, Seeley), are believed to 
have sprung from that type of stegocephalian Amphibia which 
possessed rachitomous vertebra, or with centra and intercentra. 
This division between reptiles and amphibians must have 
occurred as far back as the base of the Permian, or even in the 
Upper Carboniferous, because in the Middle Permian we find 
several orders of highly specialized reptiles, namely, the Cotylo- 
sauria, Cope, Prog ia, Baur, Dicynodontia, Owen, and 
Theriodontia, Owen, highly specialized in the so-called Gom- 
1 A paper presented in part before the British Association for the Advancement 
of Science at Toronto, and in full before the New York Academy of Sciences. 
Jan. 10, 1898. 
