EOCENE AND OLIGOCENE TITANOTHERES OF MONGOLIA 



923 



whereas the corresponding pair of upper incisors 

 (i', i^) display blunted summits or apices character- 

 istic of Protitanotherium, although the rounding and 

 convexity of the crown are not carried so far, there 

 being a distinct trace of a postcingulum; this character 

 is a convergent one with Protitanotherium and the 

 Oligocene titanotheres generally. 



The superior premolars are relatively elongate and 

 laterally compressed. Each possesses a prominent, 



The molars are also relatively elongate and narrow, 

 as compared with those of ProtitanotJierium. Thus 

 the ectolophs are relatively elongated; the parastyle, 

 mesostyle, and hypostyle are very sharp and prom- 

 inent, as compared with those in the contemporary 

 ProtitanotJierium monogoliense; the inner half of the 

 crown supports low protocones and hypocones, except 

 in m', in which there is only an extremely sessUe 

 prominence of the enamel representing the hypocone 



Figure 779. — Comparison of the superior grinding teeth of Protitanotherium andrewsi and of Protitanotherium 

 mongoliense to show the progressive development of the tetartocones in the superior premolars and of 

 the hypocones in the third superior molars 



A, Type of Protitanotherium andrewsi (Am, Mus. 20271); B, referred Protitanotherium andrewsi (Am. Mus. 20254); C, neotype of Protitanotherium 

 mongoliense (Am. Mus. 20263). About one-ha]f uatural size. Observe especially the progressive development of the tetartocones in p^-*; also 

 of the rudimentary hypocone in m^. 



elongated deuterocone, which extends backward into 

 a low, subsiding crest on which a faint swelling indi- 

 cates the rudiment of a tetartocone. This rudiment 

 diminishes in prominence as we pass from p^ to p*, in 

 which there is no trace of the tetartocone swelling. 

 Thus, as in ProtitanotJierium, the first, second, and 

 third superior premolars acquire the tetartocone 

 more rapidly than the fourth premolar. 



region, and the cingulum extends broadly around the 

 posterior end of the crown. The superior and inferior 

 premolars are harmonic in the retarded development 

 of the secondary cones, styles, and stylids. The en- 

 toconid below is represented by a ridge continuous 

 with the rudimentary metastylid. No distinct ento- 

 conid appears below, in keeping with the absence of 

 a distinct tetartocone above. 



