THE LIFE OF MAMMALS 
the order, afford a basis at once for a prime division into 
two suborders thus: — 
I. Duplicidentata, — having four upper incisors; and 
Il. Simplicidentata, — having two upper incisors. 
The first of these suborders (also termed Lagomorpha) 
contains two families of ‘double-toothed” rodents: (1) the 
pikas, small, short-eared and tailless; (2) the hares, larger, 
long-eared and tailed. 
The second, “single-toothed,” suborder contains all the 
remainder, — several hundred species. It is evident that 
here convenience, at least, requires a still further classifica- 
tion. Examining the mass, it becomes plain that this suborder 
has developed along three distinct lines, represented respec- 
tively by the porcupine, the rat, and the squirrel. 
The rodents are a very ancient race, traceable by fossil forms 
as far back as the early part of Tertiary times. The types 
mentioned above are plainly to be distinguished 
in the oldest Miocene, but the Eocene has as yet 
produced nothing but the lowest, or squirrel, type. The strik- 
ing fact about this history is that no essential change has taken 
place in the rodent type since the Miocene; and it is prob- 
able that the divisions of the Rodentia are descended from 
corresponding groups of a single primary source. This pri- 
mary source, in the opinion of Professor Cope, a high authority 
here, may have been the group Tillodontia. Other indica- 
tions point to derivation from ancestral Marsupialia, with or 
without the intervention of the Tillodontia. 
Ancestry. 
The Tillodontia are a small group of primitive mammals, fossil in the 
earliest Tertiary strata, chiefly of North America, which developed perhaps 
at first in advance of the Rodentia, and later beside them. They are very 
generalized in structure, having, for instance, in the upper jaw not only the 
three pairs of incisors of the normal mammalian set, which have been lost 
wholly or in part by the rodents, but also having definitely marked pre- 
molars. Already, however, the canines had disappeared to mere rudi- 
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