498 



TITANOTHERES OF ANCIENT "WYOMING, DAKOTA, AND NEBRASKA 



gigas, 9 , chiefly because the carpus of this specimen is 

 slightly different from that of Marsh's type of Brontops 

 robustus. Unfortunately the specimen lacks all the 

 front teeth, as far back as p' and p2. The upper pre- 

 molars have the tetartocones well constricted, as in 

 Brontops, and very different from the circular tetar- 

 tocones of male brontotheres. A supposed female of 

 Brontotherium gigas (Am. Mus. 1006) also has the 



resemblance to the jaws of brontotheres; from Bron- 

 tops robustus type it differs to some extent; perhaps 

 its nearest resemblance is to the type of Diploclonus 

 tyleri. The available measurements of the skull and 

 dentition, though few, are nearer to those of Brontops 

 robustus (especially Am. Mus. 1069) than to those of 

 large male brontotheres. The specimen is much 

 larger than the supposed female brontotheres of B. 



B 



A C 



Figure 426. — Sections and contours of skulls of Diploclonus bicornulus and D. tyleri 

 A, Diploclonus hicornuius. Am, Mus. 1476 (type); horns much as in Brontops dispar but with an accessory hornlet, basal section roundly trihedral, nasals 

 long, zygomata little expanded. B, Am. Mus. 1081, referred to i>. tyleri: stout horns roundly trihedral in section, connecting crest high, zygomata 

 stout. C, D. tyleri, Amherst Mus. 327 (type); horns widely oval in section, accessory hornlets more pronounced than in hicornutus, zygomata expanded. 

 One-seventh natural size. Sections and contours of D. bicornuius and D. selwynianus (see fig. 185) indicate that these forms, with their peculiar narrow 

 nasals, are not ancestral in type to D. ampins, with its broad and abbreviate nasals. 



tetartocones much restricted, but the reference of this 

 specimen to Brontotherium is very doubtful. 



In Am. Mus. 518 the external cingulum of the upper 

 premolars and molars is absent, as in both Brontops 

 robustus and Brontotherium, so this character is not 

 decisive. The external cingulum of the lower pre- 

 molars is reduced, but on the whole the dentition ap- 

 pears closer to that of B. robustus than to that of 

 Brontotherium. The lower jaw presents no close 



curtum and B. gigas. The sections and contours of 

 the horns and nasals are certainly different from those 

 of the supposed female brontotheres and still more 

 so from those of male brontotheres. The sections are, 

 in fact, closer to those of Diploclonus tyleri and Bron- 

 tops robustus (especially Am. Mus. 1083). The 

 manus has the magnum broader, more angulate than 

 that of B. robustus as figured by Marsh, but as a whole 

 the manus is similar in proportions to that of Brontops 



