EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 



391 



prominent supratemporal crests, which arise from the 

 postorbital processes and in some adult individuals 

 converge in the form of two broadly convex ridges 

 into the short and narrow sagittal crest (10 mm.)- 

 Between these ridges there is a median depression. 

 The suture between the frontals and 

 parietals can apparently be made out 

 in the Princeton skull (No. 10041, 

 Mesatirhinus petersoni?). The parie- 

 tals are best observed in the same 

 skull and in Am. Mus. 1509 (-M. 

 petersoni). In the superior view of 

 the skull as figured we observe also 

 the short symphyseal union (42 mm.) 

 between the premaxillaries (much 

 more abbreviate than in Dolicho- 

 rhinus), the prominent infraorbital 

 shelf on themalars, and the slender 

 section of the zygomatic arches. 



In the inferior view of the skull of 

 M. petersoni (fig. 328) we observe 

 the converging incisive borders of 

 the premaxillaries, the relatively 

 narrow and transversely arched 

 palate, which measures 152 milli- 

 meters from the incisive foramen to 

 the posterior nares. The palatal 

 portion of the palatines measures 

 70 millimeters in the midline and 

 converges anteriorly; on either side 

 of the posterior nares the convex 

 inner surfaces of the palatines con- 

 verge, and on the inner side of the 

 narrowest portion of this postnarial 

 space are placed the slender ptery- 

 goids, which are well defined. The 

 conformation of this entire region is 

 very characteristic of this genus as 

 well as of M. manteoceras . The 

 basioccipital region is best exhibited 

 in the Princeton skull (No. 10041, 

 M. petersoni), a very distinctive 

 feature being the wide separation 

 (28 mm.) by a plate of bone 

 the foramen ovale and foramen 

 lacerum medium, the same plate 

 measuring but 17 millimeters in 

 the contemporary Limnohyops lati- 

 ceps. The conformation of this important region 

 of the skull, as well shown in Figure 333, includes 

 the following noteworthy features: (1) The deep 

 groove extending backward and inward on the inner 

 side of the postglenoid facets, believed to have lodged an 

 extension of the meniscal cartilage, as in the horse; (2) 

 the prominent basioccipital and basisphenoid; (3) the 

 narrow bridge of bone between the foramen condylare 

 101059— 29— VOL 1 28 



and the foramen lacerum posterius; (4) the continuity of 

 the foramen lacerum medium and foramen lacerum pos- 

 terius; (5) the peculiar inward extensions of the con- 

 dylarfacets; (6) the general elongation of the basicranial 

 axis; (7) the open nature of the auditory meatus. 



nf One-fourth natural size, 

 tain, Henrys Fork, B 



FiGUEB 328. — Skull of Mesatirhinus petersoni 



Ai, Side view (reversed); Britisli Mus. (formerly Am. Mus. 1556), Big Bone Moun- 

 idger Basin, Wyo., Bridger D; occipital region restored from Am. Mus. 1509, Big Bone 

 Mountain, Henrys Fork, Bridger Basin, upper Bridger; and Princeton Mus. 10041; Incisors and canines 

 from Am. Mus. 1571, Washakie Basin. Az, Top view; occipital region from Princeton Mus. 10041. As, 

 Palatal view; details of pterygoid region from Am. Mus. 1509 (see above); incisors from Am. Mus. 1571 (see 

 above) and 1514 (Af. mcgarhinns) , La Clede Meadows, Washakie Basin; suture between basioccipital and ex- 

 occipital from Am. Mus. 12202 (J/. mejarUnus) , Summers Dry Creek, Bridger Basin, Bridger C 5, lower level. 



In the lateral view of the skull of M. petersoni 

 (figs. 327, 328), we observe especially the horizontal 

 suture connecting the maxillaries with the nasals, the 

 lateral compression of the sides of the face at this 

 point, the rudimentary horn convexities of the nasals, 

 the scalelike overlap of the nasals by the frontals, 

 the participation of the maxillaries in the anterior 

 portion of the infraorbital shelf, the vertical extension 



