118 



THE MERYCOIDODONTID/E 



vaulted, and the palatonarial border is well aft of the last molars. The basicranial axis is steep. 

 The glenoid articular surface is strongly oblique to the sagittal plane and is slightly convex, the 

 transverse diameter of 42 mm. being the greater, in contrast to the anteroposterior diameter of 

 18 mm. The postglenoid process is large, measuring 22 mm. in the transverse diameter and 

 17.5 mm. in the anteroposterior. The bullae are moderately large, are transversely compressed, 

 and extend as far as but not much below the level of the postglenoid processes. The paroccipital 

 processes are moderately stout, oval in section, rotated inward and forward, and in contact with the 

 bullae as well as being close to the postglenoid processes. The narrow occipital pillar is very strongly 

 convex, and the occipital condyles are large. Index: 0.548. 



Mandible: The chin profile is but very slightly concave. There is no mental tubercle, and 

 the symphysiodental angle is 40°. The angle is very prominent, being beneath the anterior part of 

 M 3 . The inferior border of the horizontal ramus is nearly parallel with the tooth row. 



Foramina: The infraorbital foramina lie above the posterior part of P 3 , and the supraorbitals 

 are 22 mm. apart. The foramen ovale is large. I do not detect a foramen rotundum. The posterior 

 palatines are not evident. 



Fie. 78. — Promerycochcerus curvidens (Thorpe). Left half, inferior view of skull. HT. Cat. No. 10997 Y.P.M. 



3/8 rut. size. (After Thorpe, 1921.) 



Dentition: Superior: The metastyle of M 3 is small, and the hypocone is noticeably smaller 

 than the protocone. There is a faintly developed internal cingulum on all the molars and on P\ 

 There is an anteroexternal pit on P 4 , and P 1 is isolated. The shape of the canine is somewhat 

 more like that of a carnivore than that of an herbivore. Inferior: P 2 and P 3 are crowded. The 

 premolars resemble closely the Promerycochcerus pattern and have advanced over the Eporeodon 

 form. The inferior canine and incisors have internal cingula. A tiny cusp is developed from the 

 cingulum between the protocone and hypocone of M 3 . The superior molar-premolar index is 

 0.808 and that of the inferior series 0.849. 



Discussion: The subgeneric name was given in allusion to the combination of characters shown 

 in this form, which seem to link it to both Eporeodon and Promerycochcerus. It differs from 

 Eporeodon and resembles Promerycochcerus in the following characters: the posterior palatine 

 foramina are not visible; the sutures are closed, although the animal is not old; the postglenoid 

 processes are robust and prominent; the bullre are large and laterally compressed but are less promi- 

 nent than the postglenoid processes, whereas in Eporeodon the bullae are very large and the post- 

 glenoids always relatively very small; the condyles are heavier, of different shape, and more widely 

 separated at the basion; in Eporeodon the highest point of the zygoma is near the middle of the 

 temporal fossa, but in this species the squamous part rises aft, as in the other Promerycochceri, above 

 or just in advance of the glenoid articular surface; the infraorbital foramen is above the posterior 

 part of P 3 , while in Eporeodon it is normally above the anterior part of P 3 and in the majority of 

 species of Promerycochcerus above P 4 or above the interval between P 4 and M 1 ; the basicranial axis 

 is steep, a condition frequently found in Promerycochcerus but not in Eporeodon; P 1 is isolated. 



