938 



EOCENE AND OLIGOCENE TITANOTHERES OP MONGOLIA 



exhibits a rudimentary tetartocone spur; p' (ap. 29 

 mm., tr. 41) exhibits a promment and distinct tetarto- 

 cone; p* (ap. 34 mm., tr. 51, estimated) has a prominent 

 and distinct tetartocone. 



The partly crushed molars, m'~^ measure 208 

 millimeters on the left side, probably elongated but 

 crushed; the approximate proportions are, m-, antero- 

 posterior 74 millimeters, transverse 64, with subequal 

 protocone and hypocone, ectoloph sliglitly cingulate; 

 m', anteroposterior SO millimeters, transverse 72 



FiGtJEE 792. — Referred jaw of Brontops gobiensis (Am. Mus. 20353) 

 and type right second lower grinding tooth of Menodus mongoliensis 

 (Am.Mus. 20351) 



Jaw one-fourth natural size, grinding tooth nine-eighths natural size. A, Superior aspect 

 of referred symphysis of Brontops gobiensis: Ai, inferior aspect of same symphysis; Aj, 

 left lateral aspect of same symphysis; B, superior aspect of type right second lower 

 grinding [tooth of Menodus mongoUevsis; Bi. internal aspect of same tooth; B2, external 

 aspect of same tooth. 



(estimated), without hypocone, a prominent cingulate 

 shelf. 



Other type characters and measurements may be 

 derived from Figure 791 and from the occiput as fig- 

 ured in Figure 775, A. 



Comparison with Brontops hrachycephalus. — The type 

 cranium of Brontops gobiensis indicates an animal of 

 approximately the same size as the larger specimens 

 of South Dakota referable to Brontops bracTiycephalus, 

 namely. Am. Mus. 1495 and Am. Mus. 4259. 



The skull of B. gobiensis of Mongolia is less pro- 

 gressive than that of the American B. bradiycephalus 

 in retaining the extremely elongate, shovel-shaped 



nasals characteristic of ProtitanotJierium; also in the 

 elongation of the bases of the horn cores. B. gobiensis 

 is also conservative in retaining three incisors above, as 

 in the upper Eocene American and Mongolian species 

 of ProtitanotJierium. Brontops gobiensis is progressive 

 and allies itself with the American species of Brontops 

 in its extreme brachycephaly. The tetartocones in 

 the superior premolars (p', p^, p*) of B. gobiensis are 

 decidedly more progressive than those in the referred 

 skull of B. brachycephalus (Am. Mus. 4259). Con. 

 sequently, although we regard B. gobiensis as more 

 nearly referable to the genus Brontops than to 

 any other genus of American titanotheres, it is 

 clearly distinguishable from members of the Bron- 

 tops phylum beginning with B. brachycephalus and 

 ending with B. robustus, of the upper titanothere- 

 bearing beds (Chadron formation) of South 

 Dakota. 



Referred jaw and symphysis (Am. Mus. 20353, 

 fig. 792, A, A], A2). — A jaw from the Ardyn Obo 

 formation, collected in 1923, is somewhat doubt- 

 fully referred to the species Brontops gobiensis, be- 

 cause it is extremely massive and because the 

 LQcisive, canine, and anterior premolar borders 

 are poorly preserved. The approximate measure- 

 ments are: 



Millimeters 



Length of symphysis 146 



Width of symphysis 170 



Depth of symphysis 84 



Width of ramus 67 



Other characters of this jaw are clearly dis- 

 played and figured in Figure 792. 



SUBFAMILY MENODONTINAE 

 Menodus Pomel, 1849 



The generic characters of this long-headed 

 titano there as it appeared in Mongolia are de- 

 rived from the single type molar tooth discovered 

 in the Ardyn Obo formation, of lower Oligocene 

 age. In contrast to that of Brontops, this tooth is 

 long and narrow and has elevated or hypsodont 

 crowns, prominent highly sculptured cingula, sharp 

 serrate ridges descending from the internal cusps, 

 broadly open valleys of the trigonid and of the 

 talonid. This form of lower grindiag tooth, which 

 constitutes the type of Menodus mongoliensis, belongs 

 with an elongated cranium, an elongated grinding 

 series, deep, narrow mandibular rami, deep and not 

 broadly expanding zygomatic arches, and elongate 

 and relatively slender limbs and feet — a harmonic 

 group of cranial, dental, and skeletal characters as far 

 as possible from the Brontops form. 



The existence of a species of the genus Menodus in 

 central Asia is apparently confirmed by the presence 

 of a related species discovered in Bohemia, namely, 

 Menodus ( = Titanotherium) bohemicus Kiernik. 



