76 : Bulletin American Museum of Natural History. [Vol. XXXIV, 
Distinctive characters: m!~* = 25, mis = 29 mm.; heels of molars small, trigonids 
high, enamel nearly smooth, jaw long anteriorly deeper and more massive than in ° 
S. strenua, pi two-rooted, pe spaced, p34 with distinct anterior basal cusps, molars 
increasing but little from m,; to m;. Posteroexternal cusps of p* and p‘ strong, 
antero-external cusps minute or absent. Metastyle of m! moderately extended; 
m? wide transversely, its antero-posterior diameter comparatively small, pr and me 
of equal height and rather closely connate, ps and ms subequal, extending more 
- externally than in other species. Parastyle of m? much extended externally, para- 
cone high, metacone almost vestigial. 
This species is close to T. white on the one hand, to S. strenua on the 
other, representing an intermediate stage in character of teeth, but larger 
than either species. 3g 
No. 16214, lower jaws and fragments of the skeleton from the top of the 
Almagre horizon of the New Mexican Wasatch agrees fairly well with the 
fragmentary jaws of the type specimen, and is taken as a topotype. No. 
16821, upper and lower jaws with parts of skull and skeleton from the upper 
level of the Gray Bull horizon of the Big Horn. Wasatch, agrees with the 
topotype. The distinctive characters above noted are chiefly based on 
these two specimens, the type having no teeth preserved. 
No. 12776, upper and lower jaws from the Lost Cabin beds in the Wind 
River Basin is also referred here. 
A comparison of No. 16821 with Tritemnodon agilis shows a close agree- 
ment in details of skeleton construction, so far as comparison can be made, 
but the Wasatch species is more primitive in the following particulars. 
The astragalar trochlea is less grooved, its inner crest less defined, and 
it is more limited posteriorly, the astragalar foramen more distinct. The 
head of the astragalus is wider and of somewhat less depth. The astragalo- 
calcanear facet is wider. The calcaneo-cuboid facet is wider; the calcaneo- 
fibular facet less extended backwards, the tuber calcis somewhat heavier. 
The astragalar facet of the tibia is somewhat flatter and more oblique, the 
internal malleolus has a more prominent posterior tuber and less prominent 
anterior crest. The third trochanter of the femur is considerably further 
down upon the shaft. The skull and jaws are more robustly proportioned, 
with a remarkably long sagittal crest great overhang to the occiput, long 
postorbital region and contracted brain-case. We have no good skull of 
I’. agilis in the collection, but S. hians differs in these skull features from: 7’. 
white and more markedly from Wortman’s reconstruction of I’. agilis or 
from Sinopa grangeri of the Bridger. 
The above specified differences also separate S. hians from the Middle 
Kocene species of Stnopa (S. rapax, grangert). It appears therefore that the 
evolution of the Hyznodonts from Lower to Middle Eocene was In part 
parallel progressive. 
