56 THE AMERICAN MUSEUM JOURNAL 
The Dromatherium 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 Paleozoic 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 
modern races of mammals, as we trace them back towards their 
1 Exhibited in the Hall of Fossil Reptiles, south side. 
