922 



EOCENE AND OLIGOCENE TITANOTHERES OF MONGOLIA 



the angle; (3) the relatively shallow rami which 

 decrease ia depth anteriorly, passing into the laterally 

 constricted diastema borders which expand again into 

 the broadened borders for the cutting teeth, a char- 

 acter similar, to that which gave the specific name 

 hyognathus (suilline-jawed) to the type from the 



Dental characters. — The dentition of Z>. Icaiseni, as 

 distinguished from the more ancient, primitive, and 

 slender DolichorJiinus olseni of the Irdin Manha, is dis- 

 played in the aged type (Am. Mus. 20252), in the fully 

 adult referred skull and jaws (Am. Mus. 20255), in 

 the somewhat less adult referred skull and jaws (Am. 



1'=^^^"-^:: 



^ 



{S^t 



FiGUHE 778. — Internal aspect of the inferior grinding teeth of five species of 

 titanotheres from upper Eocene beds of Mongolia, Burma, and Utah, in 

 descending order 



A, PTOlitanotherium supcTbum, Am. Mus. 2501 (type), Uinta formatiou, Utah (horizon C of Uinta 

 Basin); B, PTOlitanotherium mongoliense, Am. Mus. 18653 (type), Shara Murun formation, Mongolia; 

 C, Protitanotherium emaTgi-natum, Princeton Mus. 11242 (type), Uinta formation, Utah; D, titano- 

 there of an undetermined species (Am. Mus. 20014), Pondaung formation, Burma; E, Protitanothe- 

 rium grangeri, Am. Mus. 20103 (type), Irdin Manha formation, Mongolia. Observe especially the 

 progressive evolution of the talonid and the ascending rectigradations in the premolar teeth, p2-i, of 

 Protitanotherium. Protitanotherium mongoliense of the Shara Murun formation shows a marked ad- 

 vance upon P. grangeri of the Irdin Manha in the development of the entoconid; P. superbum of the 

 Uinta C is somewhat more progressive than P. mongoliense of the Shara Murun. All figures one" 

 third natural size. 



Washakie Basin of Wyoming, as named by Scott and 

 Osbom. 



It is this intermingling of the characters observed in 

 the American specimens with those discovered in this 

 Mongolian species of Dolichorhinus that gives these 



imals such exceptional interest. Other details of 

 character in the four aspects of the cranium, lateral, 

 superior, palatal, and occipital, are well portrayed ia 

 the accompanying figures. 



Mus. 20260), and in the paratype (Am. Mus. 20257) 

 of similar age and is singularly uniform in its progres- 

 sive evolution. 



The incisors diminish in size from If to I^, from 

 the lateral to the median. AH the inferior incisors 

 exhibit broad, postcingulate, almost cup-shaped bases, 

 summits constricting rapidly, crowns diminishing in 

 height from If to I-^. A pair of lower incisors (ii, 12) 

 display this cup-shaped, postcingulate character, 



