4 Origin of the Dentition of the Rodentia. 
process, we may be excused for thinking the same to have been the 
case here, although without positive tangible evidence in the shape 
of intermediate fossil forms that exhibit such a passage from the 
ordinary type.” In 1852 I had the pleasure of discovering a genus ' 
(Psittacotherium Cope) which supplies the desideratum wanting 
when Professor Ryder wrote. This is a genus, without diastema, 
and with two effective rodent-like incisors in each ramus of the 
lower jaw. Ectoganus Cope is probably similar in these respects, 
but only its separate teeth have been found. Psittacotherium is, 
then, a generalized type, 
and is not far from, if 
not.directly in, the line 
of the ancestry of all 
Rodentia. It belongs 
“a, to the Puerco fauna, 
$ which embraced so many 
of the progenitors of — 
later Mammalia (Fig. 
1 
I have called atten- 
tion to the fact that the’ 
first inferior incisor is 
FıG.1. Psittacotherium multifragum Lo e, left man- i i 
dibular ramus, one-half natural size, Original: from rudimental in Calamo- 
i 
tyne ll seabed ae Mexico, Fig. a, external view; don, and Marsh has 
shown the same thing in 
Tillotherium. In both genera the second incisor is the effective 
tooth. The thirdis present in Calamodon (Fig. 2). In Tillotherium 
the third incisor is apparently wanting. In Psittacotherium the 
first incisor tooth is present and effective, but the second is larger. 
Tt is not certain whether these are first and second, or second and 
third incisors. If we allow Esthonyx to decide the question, the 
large second tooth is truly the second incisor, for in that genus the 
first incisor is small, and the third is rudimental. With present 
information, then, the inferior incisor of the Rodentia is the second 
of the Mammalian series. 
1 American Naturalist, Feb., Tertiary Vertebrata, 1885, p. 195. 
2 I have regarded (Natu ralist, 1884, April and earlier) the Tzeniodonta 
as the ancestors of the Edentata. The objection to this view is the sup- 
posed absence of inferior incisors in the latter. But the middle incisors 
