170 
EODENTIA. 
In a third individual, the large lower incisor gives the following dimen¬ 
sions : 
M. 
Antero posterior diameter. 0. 030 
Transverse diameter at the border of the enamel «... ... p. 018 
Fragments of incisor teeth of several other individuals display similar 
proportions. The species was about the size of a Tapir. 
IIODENTI A. 
Remains of Rodents are not abundant in the Eocene beds in that part 
of New Mexico examined by me. They represent but three species of the 
single genus Plesiarctomys, which is the characteristic form of the Bridger 
beds of AVyoming. 
PLESIAEOTOMYS, Bravard. 
Plesiarctomys, Bravard, Ossemeus fossiles de Desbruge, 1850, p. 5.—Gervais, Paleonto- 
logie frangaise explic., tab. 30, ji. 4. 
fParamys, Leidy, Report U. S. Geol. Surv. Terrs., 4 to 7 i, 1873, p. 109; Proc. Pbila. 
Acad., 1870 (name only). 
The inferior molars by which this genus has been generally known 
resemble much those of existing SciuridcB, but there are cranial characters 
which distinguish it from the existing forms of that family. 
The crowns of the inferior molars support four rather small and strictly 
marginal tubercles, which inclose a median valley. The anterior inner 
tubercle is more elevated than the others, and the posterior two tubercles 
are connected by a low ridge on the posterior border, which may be more 
or less tubercular on the last molar. In some of the species, the marginal 
tubercles are merely elevations of the margin, while, in others, the adjacent 
tubercles of a pair approximate, so as to form a pair of interrupted cross¬ 
crests. A maxillary bone supporting three molars displays characters of 
value. The details of the crowns of the superior molars are similar to those 
of the inferior series; except that, in the species examined, the inner tubercles 
are more unequal, the anterior occupying most of the border. There is a 
large round foramen infraorhitale exterms, like that of Isehyromys and Fiber, 
and entirel}^ unlike that of Gymnoptyelms and Sciurus, conforming in this 
respect to the forms of the extinct grou^) of the Frotomyiddi of Pomel. The 
incisor teeth are compressed, with narrow anterior face. The enamel is not 
