6 DESCRIPTION OF A SKULL OF 



ward and forward; above the same line the ascending process slopes upward and 

 inward. It also points strongly backward. Its free borders are nearly straight, 

 except near the apex, where they suddenly converge, and near their bases, where the 

 anterior border gently curves convexly upon the postorbital protuberance, whilst the 

 posterior border makes a concave curve towards the superior margin of the zygomatic 

 process. The internal face of the ramus is strongly convex. 



The entire span of the zygomatic arch is 25 cm., from inion to the farthest point 

 on the anteorbital margin, and 11.8 cm. from the inner curve of the zygomatic process 

 of the squamosal to the inner curve of the corresponding process of the maxilla. The 

 distance between the apices of the two rami of the malar is 18.4 cm. 



Superior view. — One of the most obvious differences in the three skulls is pre- 

 sented by the divergence of the temporal ridges. A comparison of Plate II with 

 Leidy's Plate II will tell this at a glance. The angle between these ridges is acute 

 in the Owen specimen, but broadly obtuse in the other. But the Dickeson specimen 

 has the same angle still more obtuse, and no specific value can therefore be attached 

 to these differences. 



Posterior view (PI. lY, Fig. 2). — Here the differences are more important. In 

 M.jeffersoni (I.e., PI. YI, Fig, 3) the outline of the inion is semicircular; in M. leidyi 

 it is decidedly polygonal, though with rounded corners. Tlie upper portion of the 

 inion is in M. jeffersoni flattened, and bordered below by a transverse crest. In M. 

 leidyi the corresponding poi'tion bulges out to form a broad tuberosity, bordered 

 below by two transverse fossse, one on each side of the vertical crest. 



Foramen magnum is transversely oval, its horizontal and vertical diameters 

 respectively 43 mm. and 34 mm. In M. jeffersoni this foramen is circular, its diam- 

 eter 34 mm. (" 16 lines," Leidy). 



Anterior view. — In comparing Leidy's PI. YI, Fig. 2, with our PI. lY, Fig. 1, 

 it should be remembered that the specimen figured by Leidy has the intermaxilla- 

 ries preserved, which ours has not. Other differences are such that it is hard to 

 tell what is of really specific importance or may be due to age or sex. 



The nasal cavity. — The internal structure of the nose is much better preserved 

 in our specimen than in either of the skulls described by Leidy. A brief description 

 will therefore be in place here. 



Behind the incisive foramen the anterior end of the hard palate is turned upward 

 and slightly inclined backward between the alveolar walls of the canine molars, to a 

 height of about 25 mm. Its upper edge is centrally produced in a triangular process 

 with acuminate apex. A nasal crest of the maxillaries commencing about 65 mm. 

 behind its anterior border, and resting on the median line of the floor of the cavity, 



