DOUGLASS: THE TERTIARY OF MONTANA 205 
characters which distinguish it from modern Monotremes are («) the possession of 
tympanic bull, and (b) distinct alveoli for cheek teeth, which appear to have been 
simple, one rooted, peg-shaped, and six in number on each side. 
Among the fossils found near the same level were Helodermoides, Ictops major, 
Ischyromys, Hyxnodon, Agriochwrus, Limnenetes, Hyracodon, and Titanotheriwm. 
PRINCIPAL CHARACTERS. 
Characters of skull_—aAll the bones of the skull wnited into one piece, and the sutures, 
for the most part, obliterated ; cranium short with nearly the form of a pear divided in half 
longitudinally, large and full im the occipital region, tapering rapidly forward to the 
rather short, low muzzle, broad and slightly convex on top; a large portion of the brain 
case behind the vertical lambdoid ridges; swpraorbital ridges far apart and but faintly 
discernible; no post-orbital processes on the temporals; zygomatic arches convplete, but 
slender ; two infraorbital foramina ; foramen magnum large with an extension wpward 
in the occipital; tympanic bulle present and well developed ; pterygoids tending to arch 
over the space between the posterior portion of the palate, and the tympanic bull, but the 
arch not completed ; premaxillaries extending beyond the anterior border of the palate, first 
diverging and then tending to converge again above. 
There are on each side only sia alveoli for tecth. These are arranged in longitudinal 
rows, and there were apparently only six simple, one-rooted tecth on each side. Only the 
last tooth is preserved and that is a minute peg, scarcely projecting below the palate but 
apparently covered with enamel. The lower surface is polished and slightly worn by attri- 
tion. The anterior tooth was much the largest, and its root evidently extended backward 
toward the orbit. 
MEASUREMENTS. 
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DerAILED DeEscrIprion. 
Dentition. —There were apparently six teeth on each side of the upper jaw. The 
anterior alveolus by its size and the backward extension above and outside of the 
other teeth shows that this contained a one-rooted caniniform tooth. The posterior 
alveolus is small and still contains one minute, simple, peg-shaped tooth on each 
side. The other alveoli appear to have each held a one-rooted tooth. On one 
side the premaxillary is nearly complete and the one on the other side is less so. On 
these anterior projections of the maxillaries there is no vestige, or suggestion of in- 
