EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 



497 



width of zygomata 667, transverse width of occipital 

 condyles 225, condyles to tip of incisors 765, molar- 

 premolar series 350, true molars 220, p'~* 137; anterior 

 enamel of canines, worn, 42; anterior diameter of 

 canines, 36. 



Teeth (Pis. XX, C, CI). — The canines are relatively 

 _.,...._ robust, obtuse, and re- 



curved, with narrow 

 posterior cingula. The 

 incisors have smooth, 

 rounded crowns of equal 

 size, two on each side. 

 The diastema behind the 

 canine is 20 millimeters 

 and forms a very charac- 

 teristic feature of the 

 type. The premolar cin- 

 gula are nearly obsolete 

 except on the inner 

 sides of p^~*; a cingu- 

 lum is faintly shown 

 on the outer side of 

 p^. The tetartocones 

 are feebly developed 

 throughout, espe- 

 cially on p*, in which 

 the deuterocone con- 

 stitutes the chief 

 inner portion of the 

 crown with a ridge- 

 like tetartocone ex- 

 tending posteriorly. 



Lower jaw (PI. CII 

 and fig. 423).— The 

 j aw is very deep from 

 the condyles to the 

 bottom of the angle; 

 there is a robust, 

 depressed angle, 

 prominent, tall, rectangular coronoid process; 

 the mental foramen is very large; the symphy- 

 sis is long (20 mm.). There is a single lower 

 incisor on either side of the lower jaw, with the 

 vestigial median alveolus of a second incisor on the 

 right side. There is a deep median cleft in the 

 incisive border. The inferior canines are heavy and 

 laterally compressed (transverse diameter 25 mm., 

 anteroposterior diameter 32). The anterior pre- 

 molars, pi, are small but bifanged on either side 

 and are ready to drop out; there is a single alveolus 

 on the left side. The external cingulum is reduced, 

 being marked only upon the valleys and anterior 

 face of the premolar-molar series, with the exception of 

 m2_3, in which the external cingulum is somewhat 

 more developed. In all titanotheres the inner side 

 of the molar crowns is entirely devoid of a cingulum. 

 The hypoconulid of ms is not very broad but has a 



FiGUBE 424. — Sections and con- 

 tours of skull of Brontops sp. 



Am. Mus. 518 (for mounted skeleton, see PI. 

 XXXV). The horn sections and zygomata 

 suggest those of Brontops robustus. One- 

 eighth natural size. 



crenulate internal crest, which is entirely absent in 

 Menodus. 



Transitional skull to B. rohustus. — In the Princeton 

 Museum is a fine skull (No. 10061, PI. XCV) which 

 in many respects is transitional to B. rohustus. It 

 belongs to a large male and was used in the first 

 published restoration of "Menodus" coloradensis bj 

 Scott and Osborn (1887.1). Although of smaller size 

 it agrees with B. rohustus in the presence of a pair of 

 smooth, rounded upper incisors, decided diastemata 

 behind the canines, and in the general conforma- 

 tion of the zygomata, retarded development of 

 the tetartocones on the premolars, absence of ex- 

 ternal cingula, and large upper and lower canines. 

 It is more primitive than B. rohustus in the presence 

 of two rounded lower incisors, in the more rounded 

 summits of the horns, and in the somewhat less ex- 

 panded zj^gomatic and nasal sections. The angle of 

 the jaw is produced backward rather than downward. 



Figure 425. — Hyold bones of Broniops sp. (A) (Am. Mus. 518) 

 compared with those of the tapir (B), black rhinoceros (C), and 

 horse (D) 

 Oblique medial aspect of right side. sJi, Stylohyal; ch, ceratohyal; bh, basihyal; ft, 

 thyrohyal; glh, glossohyal. Compare hyoid bones of Golichorhinus longiceps (fig. 345). 

 The glossohyal is lacking in Dolichorhinus and possibly in all other titanotheres. 



Ohservations on the sTcull of the mounted skeleton 

 referred to Brontops rohustus (Am. Mus. 518). — The 

 generic and specific determination of this skeleton is 

 difficult owing to the imperfection of the skull. 



It was originally referred by Osborn to Titanothe- 

 rium rohustum, but later he referred it to Brontotherium 



