The Extinct Rodentia of North America. (Aprl, 
THE DEVELOPMENT OF THE RODENTIA. 
The Rodentia, like other divisions of Mammalia, present a suc- 
cession of changes of structure in time, in the feet and in the 
teeth. The earliest known forms, as above pointed out, are the 
allies of the squirrels, members of the sub-order Sciuromor 
pha. These have the most generalized foot structure because: 
first, the trochlear structures of the humerus and tibia are notat 
all or but little developed; second, because they have five digits 
on the feet, and are plantigrade ; and third, because the fibula is not 
` coössified with the tibia. They are similarly primitive in the forms 
of the teeth, because they are rarely prismatic, and nearly always 
have long roots and short crowns. The cavy division, or sub- order 
Hystri pha, must claim the next place, but many of its 
members show a decided advance in having a limited number of 
toes, and prismatic dentition. In the third sub-order, Myomor — 
pha, the mice, etc., we first meet with the codssification of the 
fibula with the tibia. A good many genera have prismatic teeth, 
and some of them a restricted number of digits; and a few% 
them (the jerboas) even metatarsal bones codssified into a cannon | 
bone. The rabbits have the most specialized characters in all th : 
points mentioned, but they add another character which is most . 
primitive, viz., the presence of four superior incisor teeth. Ths | 
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is probably a remnant of the primitive group from which all the 
Rodentia have been derived. By the law of homologous 8 ee 
it is not probable that the divisions of Rodentia were | 
from each other, but from corresponding groups of the pui" 
P g group This divisit 
order from which they were derived as a whole. e 
may have been the sub-order Tillodonta of the Eocenes, 0f 
Rodentia may be the descendants of the Marsupialia with or 
out the intervention of that group. o ad 
The differentiation of the sub-orders of the Rodentia € k 
dates from a period at least as early as the lowest- Miocen® has 
is an important fact that the Lower Eocene (Wasatch epoch) It : 
as yet produced nothing but the lowest type (Sciuromorpha) g , 
is also true that the Puer@o Eocene epoch has, in sixty spec! aad i 
Mammalia, disclosed no Rodentia at all, while Tillodom# © 
Tzniodonta are abundant. oligo 
The Myomorpha first appear in the White River beds (978 
cene), but none with prismatic teeth occur below the J a 
epoch. The Lagomorpha, on the other hand, pr esent n : . 
