Peterson: Material Discovered in Uinta Basin. 137 



projecting transverse process, i)erforated by a canal of considerable 

 size, which again appears on the under surface of the transverse 

 process and forms a deep atlantal groove (not foramen), at the an- 

 terior base of the same process. The anterior cotyles are deep and 

 broadly excavated above and below, as in Ilyracodon, but the accessory 

 facets for the base of the occipital condyle are less developed than in 

 the latter genus. The above description also agrees with that of 

 Triplopus cuhitalis by Professor Cope. 



The axis of No. 3399, a specimen approximately of the same size as 

 No. 3007, is considerably longer than the atlas. The bone again 

 agrees with the description of Triplopus cubitalis by Cope,"^ and of 

 Hyrachyus by Scott. ^^^ Thus the bone is relatively longer and 

 slenderer than in the Rhinoceroses and suggests the axis of the horses 

 through its prominent spine and odontoid process, ventral keel, and 

 the oblique and concave posterior face of the centrum. 



The remaining cervicals, which are represented in specimen No. 

 3007, appear to gradually shorten from the axis backward, and agree 

 generally, so far as comparison can be made, with those in contem- 

 poraneous Uinta species as well as Hyracodon. 



Measurements. 



No. 3007. No. 3399. 

 Transverse diameter of anterior cotyle 39 mm. 



Vertical " " " " 22 " 



Greatest vertical diameter of atlas 33 " 



" length of axis, odontoid process not included 48 mm. 



Length of odontoid process 14 



Besides the specimens described above, there are in the Carnegie 

 Museum a number of individuals from the same locality and horizon, 

 which perhaps pertain to a third species intermediate in size between 

 Prothyracodon obliquidens and P. uintense. It is thought best to defer 

 adding more species until more complete specimens are obtained. 

 Two species of Prothyracodon from the Uinta were originally proposed 

 by Scott and Osborn, which Osborn united in 1889 (/. c, p. 525). 



I do not hesitate in expressing my agreement with earlier workers 

 (Scott, Osborn, Wortman, Earl and others), in regard to the phylogeny 

 of Prothyracodon. From the evidence at hand there is compara- 

 tive certainty that some genus closely allied to Hyrachyus should 



I'l Tertiary Vertebrata, p. 683. 



132 "Die Osteologie von Hyracodon Leidy," Festschrift fur Carl Gegcnbatir, p. 363. 



