EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 



409 



Cranium and mandible. — The cranium is somewhat smaller 

 than in the type of Dolichorhinus longiceps, the sagittal area of 

 the parietals is more compressed laterally, the zygomatic 

 portion of the squamosal is slenderer and less expanded laterally, 

 and the basicranial axis has a greater bend. These characters 

 together with the slightly larger teeth constitute the most 

 marked differences in the two crania compared, but that they 

 should be regarded as of specific value is rather questionable. 

 The base of the skull has received some crushing fore and aft, 

 a fact to which the greater curvature of the basicranial axis 

 may partly be due. 



The sudden downward bend of the occiput of Dolichorhinus 

 heierodon, the flatter frontal region, the smaller preorbital 

 ledge, and the smaller and more delicate nasals seem to separate 

 that species more widely from the present specimen. Further- 

 more, the difference in the geological horizons in which D. 

 heierodon and the present specimen were found is to be consid- 

 ered. The former came from horizon "Lower C, " while the 

 latter was found in the lower part of horizon "Upper A" of 

 the Uinta sediments. 



The high coronoid process and its sudden backward turn at 

 the top, so characteristic of the mandible of Dolichorhinus, is 

 well shown in this specimen. The angle is much compressed 

 laterally, the temporal fossa is located high up but is quite 

 deep, and the horizontal ramus has but small vertical diameter. 



Measurements Milli- 



meters 

 Length of skull from anterior border of the orbit to top of 

 occiput 365 



Anteroposterior diameter of upper molar series 125 



Transverse diameter of frontals at postorbital processes 145 



Depth of mandible at ms 71 



Length of stylohyal, approximately 168 



Anteroposterior diameter of basihyal, median line 15 



Hyoid arch. — The hyoid arch may best be compared with 

 that of the tapir, because in that genus there is apparently no 

 extended anterior appendix or process such as is seen on the 

 basihyal of the horse or the rhinoceros. However, the bone as 

 a whole, especially its anterior border, is relatively heavier than 

 in the tapir. The thyrohyal is unfortunately broken off on 

 both sides. This element was perhaps relatively less developed 

 than in Tapirus ierrestris. The ceratohyal is also unfortunately 

 broken off at the upper end, but its length was no doubt pro- 

 portionately equal to that of the American tapir, while the 

 shaft is less constricted anteroposteriorly. The epihyal is not 

 present; this bone no doubt was nodular in character, as is the 

 case in Tapirus terrestris. The anterior portion of the shaft of 

 the stylohyal is rounder in cross section than in the tapir or 

 the horse, but the upper end is flattened and terminates in 

 enlarged processes, the superior attached to the hyoidial portion 

 of the temporal bone and the inferior somewhat more obtusely 

 rounded, extending downward and outward. This riblike 

 upper end of the stylohyal is more suggestive of the rhinoceros 

 or the horse than of the tapir. (See figs. 344 and 345.) 



Dolichorhinus hyognathus (Osborn) 



[Telmatherium cornutus Osborn] 



Plates XVII, XLVI, LII, LIII, LV, LXXI, LXXII; text 

 figures 27, 33, 105, 110, 215, 217-219, 254, 255, 302, 336, 337, 

 339-341, 346-353, 483, 511, 520, 521, 579, 580, 582-585, 588, 

 647, 661, 686, 737, 743, 745 



[For original description and type references see pp. 169, 173. For skeletal characters 

 see p. 645] 



Type locality and geologic Tiorizon. — Uinta Basin, 

 Utah; summit of Eohasileus-DolicTiorliinus zone (Uinta 

 B 2). This animal is very abundant within its known 

 geologic range through the upper 200 feet of Uinta B 2. 

 The type specimen of D. cornutus ( = 7iyognathus) and 

 most of the crania in the American Museum collection 

 were found by Peterson in the upper or " Amynodon 



sandstones," at the summit of Uinta B 2, but the 

 animal has also been recorded by Peterson 150 feet 

 below the summit of B 2. It is not thus far recorded 

 in Uinta C A single specimen has been found in the 

 Washakie Basin, Wyo., on the 180-foot level of Washa- 

 kie B 2. 



Specific characters. — Skulls large, 550 by 240 to 595 

 by 285 millimeters; relatively narrow, cephalic indices 

 46 to 43; face relatively long, faciocephalic index 53 

 to 51; grinding series p'-m^, average 206 millimeters; 

 molar-cephalic index 38; horn cores very prominent; 

 face decidedly bent down on cranium — that is, cypto- 

 cephalic; secondary palate broad and nearly on the 

 same plane with the primary palate; premolar ecto- 

 lophs more hypsodont ; premolar protocone convexities 

 sharply ridged; molars with prominent cones and 

 crests; vestigial protoconules; hypocones of m^ very 

 distinct. 



Figure 344. — Skull referred to Dolichorhinus longiceps? , side 



and top views 



One-sixth natural size. After Peterson. Carnegie Mus. 2865. 



This animal, from Uinta B 2, represents the most 

 advanced stage known of this series. It appears to be 

 a progressive descendant of D. longiceps from the base 

 of Uinta B 1. The crania are somewhat larger in all 

 dimensions than those of D. longiceps or D. inter- 

 medius, and the parieto-occipital vertex is broader. 



Synonymy. — The male jaw, type of D. hyognathus 

 (Princeton Mus. 10273), was found by the Princeton 

 expedition of 1878 in Washakie B 2. When compared 

 with the female skull and jaw (Am. Mus. 13164) 

 found at the 185-foot level of Washakie B 2, it can 

 not be separated specifically. Thus they must both 

 be referred to D. hyognathus. These specimens in 

 turn closely resemble in form and measurement the 

 females in Uinta B 2 which were first referred to D. 

 cornutus. Thus D. cornutus can not be separated 

 specifically from D. hyognathus. 



