116 



THE MERYCOIDODONTID^E 



The brain case is small and elongate, with a very prominent lateral ridge. The palate is wide 

 and vaulted, with the dental parapet projecting well below, and the U-shaped palatonarial border 

 lies about midway between the posterior of M 3 and the anterior of the glenoid surface. The palatal 

 vaulting is greater in both P. macrostegus and P. chelydra. The angle of the basicranial axis is 

 steep. The glenoid articular surface is gently convex and narrow anteroposteriorly but very long 



Fig. 75. — Promerycochcerus chelydra la/ideus Thorpe. Left half, inferior view of skull. 



1/4 nat. size. (After Thorpe, 1921.) 



HT. Cat. No. 10961 Y.P.M. 



(70 mm.) transversely. The postglenoid process is very robust, with the transverse diameter the 

 greater. The bulla; are proportionally small and triangular in basal outline and extend from the 

 paroccipital processes to a line through the middle of the glenoid articular surface. The cone-shaped 

 bulla; of P. macrostegus are the smallest in the genus, and those of P. chelydra are small and sub- 

 conical. The bulla; in P. c. latidens extend to but not below the inferior edge of the postglenoid 

 processes. Above the foramen magnum there is a very strong convexity, separated from the pos- 



Fig. 76. — Promerycochcerus chelydra latidens Thorpe. Right half, superior view of skull. HT. Cat. No. 10961 Y.P.M. 



1/+ nat. size. (After Thorpe, 1921.) 



terior temporal angles by very deep lateral fossa;. The structure is similar in P. macrostegus, but in 

 P. chelydra the convexity is wide and shallow. The coossified mastoid and paramastoid (paroccipi- 

 tal) processes approach very near to the postglenoid process. The paroccipital process is triangular 

 in cross section and is robust — not nearly so slender as in P. macrostegus. It abuts directly against 

 the bulla. Index: 0.72. 



Foramina: The infraorbitals are above P 4 . The supraorbitals are 24 mm. apart. The basi- 

 cranial foramina are normal. 



Dentition: The metastyle of M 3 is large, and the tooth as a whole is more like that of 

 P. suferbus than that of P. macrostegus. P 1 is isolated by diastemata, and it, together with P 2 , is 

 unusually small. The molars and P 4 have the internal cingulum interrupted. The molar-pre- 

 molar index is 0.88. 



