EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 



441 



the teeth it is safe to infer that the proportions of the 

 skull also differed widely. 



Comparison with DolicJiorhinus : Resemblances to 

 members of this genus are observed in the flattening 

 of the premolar ectolophs, especially of the tritocones; 



A.M. 250i, type 



^YaleMus.l03Z0,type 

 FiGtTBE 371. — Upper molars of Diplacodon and 

 Prolitanotherium compared 



One-half natural size. Superimposed contours of the first and 

 second upper molars of the dolichocephalic D. elatus (heavy 

 line) {Yale Mus. 10320, type) and the brachycophalic P. su- 

 perbum (light line) (Am. Mus. 2501, type) . 



in the slenderness of the zygomatic arches posteriorly; 

 in the elongate or dolichocephalic type of the grinding 

 teeth; and in the small size of the canines. The most 

 important progressive or divergent differences from 

 Dolichorhinus are the great development of the tetar- 

 tocones, especially on p*; the great breadth of p**; the 

 absence of a broad infraorbital shelf on the malars; 

 the apparent retardation of the horn rudiments. 



Comparison with RhadinorJiinus: Some resemblances 

 between Rhadinorhinus and Diplacodon elatus led 

 to the doubtful view that the two were related. 

 These resemblances are seen especially in the propor- 

 tions of the molars, which are of dolichocephalic type, 

 and of the premolars, which are relatively broad. 

 Among other characters common to the two species 

 are the following: Molars subhypsodont; m^ of elon- 

 gate, compressed form; parastyles and mesostyles 

 sharp and delicate; hypocones of m', m^ set well in on 

 crown; external cingula delicate; deep fossettes 

 median and posterior. In the premolars we observe 

 that the crowns are relatively broad; the tetarto- 

 cones of p^, p' are somewhat progressive in B. diplo- 

 conus; the tritocone is very large on p^-p*; and the 

 medifossettes on p^-p* are deep. 



Type skull oj Diplacodon elatus. — The fractured 

 skull affords only a few characteristic features. The 

 main indications are of a dolichocephalic type, with 

 slender zygomatic arches. The premaxillary sym- 

 physis is apparently deep, measuring 92 millimeters 

 from the incisive border to the lower border of the 



nasal notch. The extent of the palatines upon the 

 hard palate was apparently rather narrow, the pos- 

 terior nares opening directly opposite the interval 

 between the second and third molars. There is the 

 characteristic rugosity at the junction of the basi- 

 sphenoid and basioccipital. As above noted, there is 

 no conclusive evidence regarding the horn rudiments 

 except that if present at all they appear to have been 

 not very prominent. 



The most important feature by far is the slender 

 and simple structure of the zygomatic arch (fig. 369). 

 The malars give no evidence of the existence of an 

 infraorbital shelf; on the contrary, this region was 

 smooth, flattened, and not very prominent. Simi- 

 larly, the squamosal portion of the arch is shown, 

 giving a maximum depth of 30 millimeters and a 



Figure 372. — Facial region of Eotitanotherium os- 

 borni and Brontotherium leidyi 



One-fifth natural size. A, E.oshorni, Carnegie IVIus. 2859 (type), Uinta 

 B; B, B. leidyi, Nat. IVIus. 4249 (type), Chadron A. 



width of 37. The very slender zygoma was apparently 

 nearly parallel with the sides of the skull, as in Doli- 

 chorhinus, but as in Rhadinorhinus diploconus it 

 lacked the infraorbital shelf; it also lacks the deep 

 vertical expansion seen in Telma.therium. 



