174 
PHBNACODUS. 
The dentition of the anterior parts of the jaws of the species referred 
by me to Phenacoclus is unknown, but the premolar and molar teeth are 
similar in character to those of the genus PalcsocJioerus of the French 
Eocene. The well-marked external crescents are, however, characteristic, 
and the details of structure of the molars so closely resemble those of 
Opisthotomus. and Tomitherium that I entertain a suspicion that this genus is 
allied to them. If so, it includes the largest of the primitive Eocene 
Mesodonta, the P. primcavus equaling the Orang in the size of its jaws. 
There are abundant specific peculiarities distinguishing the French and 
American species, the- upper molars of the latter resembling in a greater- 
degree the genus CJmropotamus. 
Phenacodus primaevus, Cope. 
Plate xlv, figs. 1-5. » 
Cope, Paleontotogical Bulletin, No. 17, 1873, p. 3; Report Vert. Foss. New Mexico, 
U. S. Ceog. Survs. W. of lOOtli M., 1874, p. 10; Id., Anu. Report U. S. Geog. 
Snrvs. W. of lOOtb M., 1874, p. 122. 
This species was first recognized by the writer in a posterior inferior ^ 
luolar of a Mammal of about the size of a Peccary, of unknown affinities, 
which was named P. primcevus. Specimens of the same species, embracing 
the dentition of both jaws, having been procured in the Eocene of New 
Mexico, I have been able to add to the characters of the genus. The 
reiuains certainly referable to it include only fragments of jaws with denti¬ 
tion of six individuals, the most perfect embracing five superior and four 
inferior molars, all found embedded in the rock in immediate juxtaposition. 
A detailed description of this specimen is given below. 
The penultimate superior premolar, viewed from below, exhibits the 
outline of a right-angled triangle, with the short base anterior, and directed 
at a right angle to the exterior side. The outer portion of the crown consists 
of a rather low compressed cusp, marking the posterior third of the length. 
The edge whicli connects it with the anterior base, is interrupted at its 
middle by a low tubercle. The inner part of the crown is produced into a 
stout, prominent cusp, which is opposite to the anterior outer tubercle just 
described, and is se})arated from it by a Avide longitudinal valley. There 
are no other tubercles, and no basal cingula, except a trace at the posterior 
