Marsh Collection, Peahody Museum. 



135 



genus, and is therefore the only Primate thus far known from 

 the Uppermost Eocene of North America. 



Its relationship is at once seen in the elevated character of 

 the crown of the third premolar, as w r ell as in the general 

 agreement in the structure of the teeth. It is, however, the 

 largest species of the genus known, and exhibits a marked 

 advance in the structure of the teeth, in the more widely sepa- 

 rated and distinct condition of the internal cusp of the fourth 

 premolar, as well as in the absence of the anterior cusp of the 

 trigon on the second molar. The Bridger species are all 

 smaller and more primitive. 



Hemiacodon gracilis Marsh. * 



Hemiacodon gracilis Marsh, this Journal, September, 1872, Separata, 

 August 13, 1872, p. 21 ; Omomys gracilis Osborn, American Eocene Primates, 

 Bull. Amer. Mus. Nat. Hist., June, 1902, p. 173. 



This is one of the most abundant species of monkey in the 

 Bridger formation, and as far as the specimens show is con- 

 fined to the upper levels of the horizon. The type upon 



128 



Figure 128. — Portion of right mandibular ramus of Hemiacodon gracilis 

 Marsh ; inside view ; two and one-half times natural size. (Type of the 

 genus and species.) 



The elevation of the summit of the crown of the third premolar is greater 

 in the type than in other specimens, on account of being partially out of the 

 socket. " 



which Professor Marsh established the genus and species con- 

 sists of a considerable part of a right mandibular ramus, figure 

 128, bearing the third and fourth premolars and the three 

 molars in excellent preservation. The specimen also exhibits 

 the alveoli for the second premolar, canine, and the two inci- 

 sors, but is not sufficiently complete in front to admit of a 

 determination of the number of incisors beyond all question. 

 As compared with Omomys Carteri, the teeth of the lower 

 jaw display in their structure a striking similarity to those of 

 this species, and it is not at all surprising that Professor Marsh 

 should have referred the two to the same genus. The chief 

 differences consist in the enlargement of the first incisor and 

 the reduced condition of the second incisor, canine, and second 

 premolar, as well as in the better development of the internal 

 cusp of the fourth premolar in Hemiacodon gracilis. The 

 relations of the teeth in the front part of the jaw, I regard as 



