EPOREODON 77 



Specific Characters: The mesocephalic skull is the same length as that of E. longifrons, 

 these two sharing the honors of being the largest of the genus described to date. The zygomata 

 attain their maximum expansion just posterior to the zygomatic foramen. The malar portion is 

 deep, but the squamosal part of the zygoma is light and rises abruptly posteriorly. The nasals are 

 long and have been partly restored. They appear to be ellipsoidal in shape and posteriorly trun- 

 cated. The pars facialis of the lacrimal bone is moderately large, with a small but deep fossa. 

 This fossa is crushed in and appears to be deeper than it probably was in reality. The frontals are 

 very flat, with but a slight supraorbital convexity. There is some restoration in the frontal region, 

 and I believe there is some downcrushing. These bones probably extended in advance of the 

 lacrimals. The orbits are small and round. The temporal ridges unite nearly above the postorbital 

 constriction to form a moderately long and thin but not high sagittal crest, nearly a third of the 

 skull length. The supraoccipital crest is posteriorly well produced, and the wings are mediumly 

 open. The brain case is narrow and elongated, with lateral convexities along the suture. The large, 

 tubular external auditory meatus is directed strongly backward and upward. The basicranial axis has 

 a very gentle slope. The palate is wide, nearly flat anteriorly but vaulted aft, and the U-shaped 

 palatonarial border is not far behind the last molars. The glenoid articular surface is not large and 

 is gently convex. The postglenoid process is likewise small, with an oblique external surface. The 

 paroccipital processes are transversely wide at the base, triangular in median section, and in contact 

 with the medium-sized bulke, and they extend downward and outward. Index: 0.58. 



Mandible: The symphysiodental angle is 39°, and the chin is moderately concave in profile. 

 The mental tubercle is slightly developed. The angle is well developed. The coronoid process is 

 small and low, the sigmoid notch is shallow and wide open, and the condyle is normal and some- 

 what inset from the very straight posterior mandibular border. 



Foramina: The infraorbitals are above the posterior part of P". The supraorbital foramina are 

 about 20 mm. apart. The foramen ovale is very large, and there is no trace of a foramen rotundum. 

 The posterior palatines are opposite P\ 



Dentition: The incisors are small. In the anterior superior premolars the posterior crescent 

 is slightly less developed than is normal for the genus. P 2 is somewhat obliquely inserted in the 

 maxillary, and P 1 is large and oblong. The superior molar-premolar index is 0.87 and that of the 

 inferior series 0.86. 



Skeleton: This is about a fifth larger than that of E. socialis. The limbs, in their general 

 proportions, resemble those of the latter species, except that the proximal elements are somewhat 

 more elongated. There are seven lumbars present in this specimen and five sacrals, the first four of 

 which are coossified. 



Eporeodon major hybridus (Leidy) 1869 

 Fig. 38 



Original Reference: The extinct mammalian fauna of Dakota and Nebraska. Jour. Acad. Nat. Sci. 

 Phila. (2), VII, pp. 105-106 (Oreodon hybridus). 



Type Locality: White River, South Dakota. 



Geologic Horizon: Oligocene (probably lower upper Brule). 



Types: Holotype, Cat. No. 10860 A.N.S.P., median section of a skull. Plcsiotype, Cat. No. 12609 Y.P.M., 

 skull. 



Specific Characters: The skull is about the length of that of Eporeodon socialis, that is, 

 somewhat shorter than E. major but wider than either across the arches. The maximum zygomatic 

 expansion is on a line with the posterior part of the zygomatic foramen. The malar is robust, and 

 the parts of the arch which are preserved correspond with the condition in E. major. The wide 

 nasals are posteriorly truncated, while in the latter species these bones are posteriorly pointed. The 

 wide, unreduced frontals rise gently toward the rear to a greater degree than in the latter. The 



