PROMERYCOCHCERUS 1 1 5 



Discussion: Two skulls in the Marsh Collection, Cat. Nos. 10962 and 10979 Y.P.M., show 

 the characters of this species very well. In general proportions and appearance they differ 

 but slightly from P. superbus, except in the much greater bizygomatic diameter characteristic of 

 P. chelydra. 



Cope (1884B, p. 523) wrote that "a line drawn through the postglenoid and paroccipital 

 processes makes an angle of 90° with the middle line, as in M. superbus." This statement must 

 have been a slip of the pen, as the angle is 60°, the same as it is in P. superbus. 



Promerycochcerus chelydra latidens Thorpe 1921 

 Figs. 74-76 



Original Reference: John Day Promerycochaeri, with descriptions of five new species and one new 

 subgenus. Amer. Jour. Sci. (5), I, pp. 232-235, figs. 2A, B, C (P. latidens). 



Type Locality: North Fork, John Day River, 15 miles from its junction with the main stream, Oregon. 



Geologic Horizon: Lower Miocene (upper John Day). 



Type: Holotype, Cat. No. 10961 Y.P.M., skull, of which the portion anterior to P 1 is broken away. The 

 teeth are much worn and the sutures nearly obliterated, due to old age. Collected by William Day in 1875. 



Specific Characters: The skull is about the length of that of P. macrostegus or of the larger 

 type of P. thomsoni — as wide as the latter and more than 1 3 per cent wider than the former. The 

 muzzle is compressed. The malar below the orbit is flat and directed somewhat obliquely outward 

 and downward. It is very deep, while the depth in P. macrostegus is 38.5 mm., in P. superbus 

 38 mm., and in P. chelydra 36 mm. The inferior edge is thickened, very rugose, and offset from 



Fig. 74. — Promerycochcerus chelydra latidens Thorpe. Skull. HT. Cat. No. 10961 Y.P.M. 1/4 nat. size. 



(After Thorpe, 1921.) 



the alveolar parapet a distance of 24 mm. It is continued forward as a maxillary convexity, divid- 

 ing the side of the face into two concave portions. The squamous portion of the zygomatic arch 

 extends forward to a point below the posterior third of the orbit, whereas in P. chelydra it does not 

 extend below the orbit. The external edge is heavier and rounder than in P. macrostegus. The 

 posterior part rises gradually at an angle of about 45° to the apex, which is not above a line through 

 the middle of the orbit. The nasals are anteroposterior^ straight and transversely convex. The 

 frontonasal sutures are obliterated, as are the sutures of the pars facialis of the lacrimal bone. The 

 lacrimal fossa is well marked and blends into the upper lateral concavity of the muzzle. The wide 

 unreduced frontals rise gradually aft and are moderately flat, except for the decurved supraorbital 

 area. The orbits look chiefly outward but somewhat upward as well. They are characteristically 

 small, with the vertical diameter greater than the transverse. They are situated high on the skull, 

 on account of the great depth of the malar bone. The rugose temporal ridges sweep back in a nearly 

 straight line to unite above a plane through the postglenoid processes. The sagittal crest is high but 

 short, not more than a fifth of the skull length. The supraoccipital crest is produced well beyond 

 the occipital condyles and has widespread wings. 



