136 



THE MERYCOIDODONTIMi 



is close to the incisor border. In all the other species the proximal sides of these foramina are 

 parallel to the sagittal plane, instead of being markedly oblique, as they are here. 



Dentition: The metastyle of M 3 is small, and P 1 is markedly isolated. The lower premolars 

 are very slightly crowded. There is no anteroexternal pit in P 4 . The superior molar-premolar 

 index is 0.89, while that of the lower is 0.88. 



Discussion: The face in the type specimen is narrowed in advance of the orbits relatively 

 more than in any other species, considering its size. 



Fig. 92. — Promerycochcerus marshi Thorpe. Skull and jaw. HT. Cat. No. 10999 Y.P.M. 1/4 nat. size. (After 



Thorpe, 1921.) 



Fig. 93.- — Promerycochcerus marshi Thorpe. Right half, superior view of skull. HT. Cat. No. 10999 Y.P.M. 



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



While this species shows similarities to P. macrostegus, yet its orbits are lower, smaller, and 

 rounder j the malar is much wider and heavier; the nasals project farther forward; the face is 

 much deeper above the premolars; the sagittal crest is very much shorter; the zygomatic crest is 

 lower, and the outer margin of the process is less robust; the occipital pillar is more convex, and the 

 lateral fossa; are deeper; the mandible is heavier and the masseteric fossa very much deeper; the 

 infraorbital foramen is smaller, and the incisive foramina are nearer together, larger, differently 

 shaped, and much nearer the incisive border. 



The dentition of P. marshi shows a much greater conformity in the superior and inferior ratios, 

 whereas in P. macrostegus the superior premolar series is relatively shorter and the inferior series 

 relatively longer in proportion to the molar series. 



