346 O. C. Marsh—American Jurassic Mammals. 
The modern Placental mammals were evidently not derived 
from Marsupials, as is generally supposed. Each group has ap- 
parently come down to the present time, by separate lines, from 
primitive, oviparous, forms, of which, the living Monotremes 
may be the more direct but specialized representatives. Among 
the diversified members of Placental mammals, the Insectivores 
are probably the nearest to the early type, and hence they show 
many features seen in the Jurassic and Triassic mammals of the 
order Pantotheria. 
Among the various existing Marsupials, the Rat-Kangaroos, 
(Hypsiprymnide) appear to be nearest to the oldest known 
forms represented in the order Adlotheria, but future discoy- 
erles may, at any time, bring to light new Mesozoic mammals 
allied to other Marsupials. 
So far as at present known, the two great groups of Placental 
and Non-placental mammals appear to be distinct in the oldest 
known forms, and this makes it clear that, for the primitive 
generalized forms (Hypotheria), from which ‘both were derived, 
we must look back to the Paleeozoie. 
Yale College, New Haven, March 16, 1887. 
