166 BULLETIN 169, UNITED STATES NATIONAL MUSEUM 



U.S.N.M. nos. 10005 and 10044, isolated upper incisors (pertinence 

 dubious). Collected by A. C. Silberling. 



Horizon and locality. — All known specimens from Gidley Quarry, 

 Fort Union, Middle Paleocene horizon, Crazy Mountain Field, Mont. 



Diagnosis: — Sole known species of genus. See also description 

 below. 



Discussion. — The lower dentition is crowded, with no diastema. 

 Ii is a very large, laterally compressed, nearly procumbent tooth. 

 Its crown is not known. Immediately above and behind the root of 

 Ii is a shallow, very small, and somewhat doubtful alveolus, probably 

 for a vestigial I2. The following tooth, probably a reduced canine, 

 is also small and 1-rooted. The root is slightly procumbent, the 

 rather formless crown more so, as it projects obliquely upward and 

 forward from the root. 



While Dr. Gidley did not discuss these anterior teeth, he gave the 

 dental formula as 1.1 or 0.4.3. from which he evidently considered the 

 first alveolus as doubtful and the tooth just mentioned as a first pre- 

 molar. While the question cannot be answered definitely, the formula 

 2 or 1.1.3.3 seems to me much more probable. The morphology is 

 indecisive, but in later plesiadepids PS are always lacking and in 

 probably related groups (especially the tarsioids) are apparently 

 among the first teeth to be lost, while the canine is more tenacious, 

 being still present in the upper, although not in the lower, jaw of the 

 much more advanced genus Plesiadapis and seldom or never absent in 

 the tarsioids even though it may be reduced.^* 



P2 is a simple tooth with one vertical root and a slightly procumbent 

 crown excavated on the inner side and with a small 1-cusped heel. 

 P3 has two roots and is not reduced relative to P4. Its crown is not 

 known. P4 is similar to that of Plesiadapis, but its base is more 

 quadrate, the trigonid portion is relatively shorter and higher, the 

 heel is at least as large, relatively, or a little larger, but its transverse 

 posterior crest rises to one apex, rather than two as usual in Plesiadapis. 



The paraconid is distinct on all the molars and is anteroexternal to 

 the metaconid. From Mi to M3 it is progressively closer to the meta- 

 conid and relatively smaller. The protoconid is about as high as the 

 metaconid on Mj, and on M2 is somewhat and on M3 much lower than 

 the metaconid. The talonids of Mi -2, which are considerably wider 

 than their trigonids, are simple and basined. A hypoconulid can be 

 distinguished but is poorly differentiated. There is also a poorly 

 developed cusp on the crest from the hypoconid to the posterior base 

 of the trigonid. The entoconid-metaconid crest is notched. There 

 is no metastylid. M3 is distinguished by the expansion of the posterior 

 end and the development of an elevated posterointernal rim, into 



6' Gidley gives the formula ?2.?i or o.?4.3 for the upper dentition, but this- is presumably an inference 

 as no specimens show anything more than that there were three upper molars and at least one premolar. 



