EOCENE AND OLIGOCENE TITANOTHERES OF MONGOLIA 



929 



clearly to demarcate this species of the Shara Murun 

 from the more primitive Protitanotherium grangeri of 

 the Irdin Manha formation. At the same time they 

 indicate a phase of Protitanotherium evolution some- 

 what more ancient geologically than P. andrewsi, the 

 giant species of this genus. Unlike P. grangeri, the 

 inferior premolars (type Am. Mus. 18653, referred 

 Am. Mus. 20256, 20273) exhibit very distinct entoco- 

 nids in P2-4, with distinct entoconid shelf and deep 

 talonid basin partly closed internally by the metastylid. 

 Similarly the superior premolars (skull Am. Mus. 20261, 

 referred), palate (Am. Mus. 20256, referred), neotype 

 palate (Am. Mus. 20263), right maxilla (Am. Mus. 

 20270, referred) all exhibit progressive tetartocones in 

 the second and third premolars, p^~^ and a more or less 



Dentition. — Of the cutting teeth the male superior 

 canines (Am. Mus. 20263, neotype; 20256, referred) 

 are large and powerful, sharpened at the extremities, 

 obtuse at the base, powerful weapons far surpassing in 

 size the small, recurved canines of the females (skull 

 Am. Mus. 20261, fig. 785). The superior incisors 

 (Am. Mus. 20263, neotype, fig. 790; Am. Mus. 

 20256, referred) in the males are greatly enlarged, i^ 

 retaining its supremacy, whUe i'"' do not exhibit the 

 pointed, incisiform character observed in Protitano- 

 therium grangeri but the bluntly rounded character 

 distinctive of American species of Protitanotherium 

 and characteristic of all the known American Oligocene 

 titanotheres. Thus the superior canines, premolars, 

 and incisors present marked specific distinctions from 



Figure 785. — Protitanotherium mongoliense, Am. Mus. 20261, complete female skull, including incisors, canine, and grinders. 



Collected in the Shara Murun formation 



One-sixth natural size. Tliis referred cranium was severely crushed laterally and has been largely restored. It displays the general proportions of the P. mongo- 

 liense skull and the texture and cream-white color characteristic of the Shara Murun fossils. 



well-developed tetartocone rudiment in the fourth 

 premolar, p*. The several maxillae and premolar 

 grinding series show progressive steps in the develop- 

 ment of this tetartocone of p*; in fact, the palatal series 

 (Am. Mus. 20263, shown in fig. 784) exhibits the tetar- 

 tocones of p'~* in their most rudimentary stage, 

 whereas the tetartocones in the skull (Am. Mus. 20261, 

 fig. 785) are in their most progressive stage. 



Hypocone 0/ m^. — Similar stages of superior molar 

 evolution are observed in the hypocone rectigradation 

 in the four referred specimens of superior grinding 

 teeth; (a) the hypocone is not visible in Am. Mus. 

 20261 ; (&) it is seen in its most rudimentary stage in 

 the right m' of Am. Mus. 20263 (figs. 779, 784) 

 whereas (c) in the opposite molar, left m^ of the same 

 specimen, there is exhibited a hypocone no larger 

 than a pinhead. 



Protitanotherium grangeri. The inferior incisors, how- 

 ever (Am. Mus. 20256), especially \2-z, exhibit the 

 pointed, typically incisiform character seen in P. 

 grangeri, although of nearly double the size. 



Cranium. — The marked cranial distinctions of 

 Protitanotherium mongoliense are observed in the skull 

 (Am. Mus. 20261, fig. 785) and palate (Am. Mus. 

 20263, fig. 784), consisting chiefly of the rounded, 

 simply convex, nonangulate character of the suborbital 

 zygomatic process of the malar; the zygomatic arch, 

 laterally crushed in Am. Mus. 20261, is somewhat 

 broadly expanded; the nasals retain a broad, spitu- 

 late, shovel-shaped character; the osseous horns in the 

 females (Am. Mus. 20261, fig. 785) were doubtless 

 smaller than in the males. The mandibular rami, 

 well preserved in the male (Am. Mus. 20256) and 

 probable male (Am. Mus. 20273), are very deep, the 



