158 THE MERYCOIDODONTID/E 



zygoma is not like that of Promerycochcerus , and the depth is greater than the average depth in that 

 genus, although in P. lulli the malar is nearly two inches deep beneath the orbit. The nasals are 

 much shortened and flattened. They extend but slightly beyond M 1 , and apparently they are 

 posteriorly truncated. The premaxilla? are coossified with each other and with the maxilla?, so that 

 the sutures are obliterated. This condition produces a very solid upper-jaw bone, separated pos- 

 teriorly by a wedge-shaped excavation which begins nearly opposite the front of P 2 and widens uni- 

 formly to a line opposite M 1 . The antorbital fossa is well defined, with rugosities on the margin for 

 the attachment of the levator labii superioris and orbicularis -palpebrarum muscles in front of the 

 orbit, and of the levator labii superioris ala?que nasi muscle at the junction of the frontal and maxil- 

 lary bones. These muscles control the movements of the nose and of the upper lips, and apparently 

 they were so prominent as to indicate the presence of a short proboscis or at the very least of very 

 flexible and mobile lips. From the posterior part of the nasals the skull rises very steeply to the 

 occiput. The frontals are wide, much inflated, and moderately convex in both transverse and 

 anteroposterior directions. They do not extend in advance of the front edge of the lacrimal bone. 

 The orbits are situated relatively much higher above the jaws than in Promerycochcerus but at 

 about the same depth below the top of the skull. They look more forward and upward than in the 

 latter genus. The temporal ridges extend backward and upward in a nearly straight line from the 

 postorbital bars to the occiput, which is higher than in Promerycochcerus. The extremely short 

 sagittal crest, if such it can be called, is low. The small supraoccipital crest is low and broad, with 

 widespread wings, and has a very slight overhang above the occipital condyles. 



The brain case is transversely rather wide, but anteroposterior!}' it is greatly foreshortened, with 

 a prominent convexity on either side. The external auditory meatus is located fairly high, about on 

 a line with the top of the zygomatic process and at the widest point of the mastoid plate. The palate 

 is long and apparently wider toward the back than it is in front. The palatonarial border lies well 

 behind the molars, about midway between the end of the last molars and the anterior edge of the 

 glenoid surface. The basicranial axis is not steep, because of the elevation of the occiput. The 

 glenoid surface is moderately wide and convex. The postglenoid process is relatively small and is 

 in contact with the large wing-like mastoid plates, which extend outwardly beyond the postglenoid 

 process. These mastoid plates have a total width nearly equal to three-fifths of the skull length. 

 The paroccipital processes are rather stout, extending downward and outward, and are also in contact 

 with the mastoid plates. Index: 0.85-0.87. (For brain casts, see Figs. 187-1 88.) 



In the plesiotype a heavy layer of cellular bony tissue has thickened the frontals, parietals, and 

 squamosals to form a prominent ridge along the anterosuperior border of the latter. 



In distinguishing between the male and female skulls, the former is presumably the heavier, 

 broader, and stockier, with a shorter and wider muzzle, larger canines, and more cellular tissue in the 

 frontal and cranial areas. 



Mandible: The symphysis is strong, with a symphysiodental angle of approximately 50°. The 

 general form of the ramus is similar to that of Promerycochcerus, except that the coronoid process 

 is much reduced, approaching in relative size that of Merychyus but differing from the latter in 

 being straighter and more backwardly directed from the base. As in Promerycochcerus there is a 

 strong supporting buttress, extending more than half of the length of the process, with its point of 

 origin on the internal upper edge of the mandible. The sigmoid notch is shallow, and the condyle is 

 robust. 



Foramina: The infraorbitals lie above the interval between M 1 and M 2 . The supraorbitals are 

 approximately 42 mm. apart and are in a line through the middle of the orbits. 



Dentition: The molars are less hypsodont than are those of some of the other species. The 

 superior molar-premolar index is 0.77. 



Superior: The molar-premolar series forms a continuous dental row, except for a diastema 

 before and behind P 1 . The incisors are spaced, relatively small, short-crowned, and less spatulate 

 than those of Promerycochcerus. The canine is fairly large and directed nearly straight downward. 



