134 



THE MERYCOIDODONTID/E 



Mandible: The chin profile is very concave, and the mental tubercle is very large, a condition 

 which, instead of being an individual peculiarity, as Cope suggested, is found in nearly all of the 

 species of this genus. It is, however, subject to much variation in its development. The symphy- 

 siodental angle is 43°. The inferior border of the ramus is straight below the tooth row, and the 

 angle is prominent. The condyle is transversely wide and convex and but slightly inset from the 

 posterior ramal border. The sigmoid notch is wide and shallow, the coronoid process is small and 

 everted, and the masseteric fossa is moderately shallow. 



Foramina: The large infraorbitals are above the interval between P 4 and M 1 . The supra- 

 orbitals are close together, being but 1 5 mm. apart. 



Dentition: The superior premolars are large, and the first three are two-rooted, while P 1 is 

 separated by a marked diastema on either side, the greater being between it and the canine. The 

 molars have no internal cingula. The inferior third and fourth premolars are slightly overlapping. 

 P 2 has a prominent internal vertical ridge. The true lower canine is very wide, and the inferior 

 incisors are crowded and overlap each other and the true canines. The superior molar-premolar 

 index is 0.84 and that of the lower series 0.89. 



Fig. 91. — Promerycochcerus macrostegus inflatus Thorpe. Skull. HT. Cat. No. 10233 Y.P.M. 1/4 nat. size. (After 



Thorpe, 1921.) 



Promerycochoerus macrostegus inflatus Thorpe 1921 



Fig. 91 



Original Reference: John Day Promerycochaeri. Amer. Jour. Sci. (5), I, pp. 235-236, fig. 3. 

 Type Locality: Bridge Creek, John Day River, Oregon. 

 Geologic Horizon: Lower Miocene (upper John Day). 



Type: Holotype, Cat. No. 10233 Y.P.M. , skull, lacking portion of basicranial and cranial area. Collected 

 by Thomas Condon. 



Specific Characters: This is one of the largest skulls known in the genus, and in length it 

 is next to the longest. It is very robust and massive and is peculiar in that the bone of all parts of 

 the skull is slightly spread apart and the interstices are filled with matrix — the character which sug- 

 gested the specific name. The maximum zygomatic expansion is just in front of the glenoid surface. 

 The malar is deep and very robust, with the anterior zygomatic pedicle originating above the 

 anterior part of M" and continuing as a convexity that occupies nearly the whole side of the face. 

 The squamous portion of the zygomatic arch is proportionally very light, with its posterior part 

 rising no higher than a plane through the middle of the orbit and its anterior part extending forward 

 to beneath the postorbital bar. The nasals are long and wide and appear to be posteriorly trun- 

 cated. The lacrimal fossa; are very shallow. The flat frontals are very wide and are unreduced. 

 The orbits are medium-sized and look mainly sidewise. The temporal ridges unite nearly above 

 the anterior of the glenoid surfaces and then form a thin, high sagittal crest, about a fourth of the 

 skull length. The brain case is very wide and low and is much the largest in any species of this 



