Vol. 4 J Sinclair. — John Day Rodents and Ungulates. 139 



toward the frontal suture. Anteriorly there is a strongly convex 

 swelling on either side of the median line, inclosing a deep de- 

 pression which extends forward from the supraorbital foramina. 

 The sagittal crest is thin and rises in a gentle convexity above 

 the interorbital plane. The superciliary borders are moderately 

 elevated. The premaxillaries are decurved to about the same ex- 

 tent as in Dicotyles. In the type specimen the maxillary is 

 strongly rugose anterior to the base of the jugal arch. This is 

 hardly noticeable in No. 1988. 



Mandible. — The rami have about the same depth from P 2 to 

 M.., tapering anteriorly. The long and narrow symphisial region 

 is inclined about thirty degrees to the axis of the jaw. The 

 mental foramina are the same in number and relative position 

 as in T. decedens. 



Dentition. — The first superior incisor has a much broader 

 crown than the second and third. The principal wear is on the 

 tip and outer side of the crown. I 2 is broken, but the cross- 

 section of its roots shows that it is a small tooth. I 3 is long and 

 pointed and considerably compressed laterally. The upper ca- 

 nines are oval in cross-section at the base, but become triangular 

 where the anterior edge is exposed to the wear of the lower ca- 

 nine. P 1 is a simple crowned, double-rooted tooth separated 

 from the canine and P 2 by diastemata. Unlike T. decedens, P 1 

 is in this species situated entirely posterior to the canine, and is 

 not overlapped by the posterior border of that tooth. It has 

 been broken off on the side seen in the figure. The remaining su- 

 perior premolars are unspaced and resemble the corresponding 

 teeth in T. decedens. The upper molars increase very slightly 

 in size posteriorly. On M 3 the heel is narrow and equally promi- 

 nent on the outer and inner sides of the crown, giving to the pos- 

 terior border of the tooth an even convexity. External, anterior 

 and posterior cingula are present on P 4 and the molars. Internal 

 cinguia are feebly developed or wanting. 



In the inferior series, the incisors are inclined forward in 

 conformity with the slope of the symphysis and are worn princi- 

 pally on their posterior faces, although the tips of the first pair 

 are abruptly truncated. The lower canines are triangular in 

 cross-section, somewhat recurved, and have a strongly marked 



