484 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



Brontops iracJiycephalus is distinguished from its 

 successor, B. dispar, by the much smaller size of all 

 parts of the skull. From Allops marshi it is distin- 

 guished by its somewhat smaller dimensions. From 

 Menodus proutii it is distinguished especially by its 

 shorter molar series and relatively wider premolars, 

 with less progressive tetartocones. These differences 

 are shown below. 



Measurements of skulls of Brontops, Allops, and Menodus, in 

 millimeters 



A female skull (Nat. Mus. 4947) that is referred to 

 B. hrachycepTialus differs from the male skuUs in its 



Figure 415. — Upper canines and incisors of Brontops braohycephalus 



Nat. Mus. 4947, 9 1 canines subconic witti slender cingula; incisors (i', i-) small, with circular and 



rounded crowns. Natural size. 



smaller size and lesser width throughout. From the 

 type of AUops walcotti this female skull differs in its 

 smaller size, especially of p'-m^ (253:285 mm.). In 

 the males of B. bracliyceplialus the fourth premolar is 

 narrower than in the species of AUops. 



Materials. — The specific characters of B. hracTiycepJi- 

 alus rest upon exceptionally complete material, most 

 of which is in the National Museum. The type of this 

 species is the adult male skull Nat. Mus. 4261 (PL 

 LXXXVII), from the base of the lower Titanotherium 

 zone. The paratype is the exceptionally aged female 

 skull (Nat. Mus. 4947; Pis. LXXXIII, LXXXVII) 

 foimd by Hatcher at the very base of the Titanotherium 

 zone only 14.4 feet above the Cretaceous Pierre shale. 

 A third specimen is skull Nat. Mus. 4940, also re- 

 corded from the lower beds but from the upper level 

 (A 3). A jaw (Am. Mus. 4247) agrees closely in size 

 with this skull but may belong with Allops walcotti. 

 A fourth specimen (Nat. Mus 1214, fig. 418) also 

 comes from the upper level of A, or the lower beds, 

 and it shows features of approach to B. dispar. A 



fifth skull (Am. Mus. 1495) is distinguished by its 

 very small horns. 



General characters. — These small, broad-skulled ti- 

 tanotheres are distinguished by the lateral arching and 

 relatively broad section of the zygomatic buccal proc- 

 esses. The nasals taper toward the extremities and 

 are slightly decurved. The horns rise above the skull 

 from 25 millimeters in the females to 60 in the male 

 skull, the corresponding outside measurements beiug 

 85 and 102. The basal section of the horns (fig. 414) 

 is highly characteristic, consistiug of a broad outer 

 maxillary face, lying in an oblique plane, and a broad, 

 highly convex inner face. The horns are obtusely 

 roimded at the top. The superior iacisors (fig. 415) 

 are two in number on each side in yoimg animals, 

 but the iuternal pair, I^ are variable and likely to dis- 

 appear in aged animals. The canines are elongate, 

 pointed, with a posterior cingulum only and a strongly 

 convex posterior face, varying in length from 33 

 millimeters in the females to 43 in the males. A 

 very distinctive character is the structure of the 

 premolars, the main internal portion of the crown 

 being composed of the deuterocones, while the tetar- 

 tocones are small and rudimentary, especially in p^ p*. 

 In the type male skull the tetartocone of p^ 

 on the right is a prolongation of the deutero- 

 cone, concave and continuous with the cin- 

 gulum on the lingual or internal border; on 

 the left side the tetartocone is more circular. 

 In Am. Mus. 1495 the tetartocone on p* is a 

 concave spur from the cingulum; the tetar- 

 tocone on p' is concave on the lingual side, 

 convex on the buccal side, which is evidence 

 that the buccal side develops faster. The pre- 

 molar cingula are developed progressively in 

 ascending levels but are never so broad as 

 in Menodus. A primitive feature of m^ is 

 the aborted metaloph extending inward upon the 

 crown. There are incipient protostyles on the superior 

 molars. 



Paratype jemale sTcull. — The very aged female skull 

 (Nat. Mus. 4947) in the eleventh stage of growth, dis- 

 covered by Hatcher in 1887 on the bottom level of the 

 lower Titanotherium zone, is the paratype of this 

 species. The palate is finely preserved (PL 

 LXXXVII). It is by far the smallest and one of the 

 most primitive skulls of the brachycephalic series; it 

 measures only 590 millimeters from the occipital con- 

 dyles to the premaxillae and 380 millimeters across 

 the zygomata, while the total premolar-molar series is 

 only 253 millimeters as compared with 320 millimeters 

 in the lower jaw of the type of Teleodus amis. The 

 nasals are rather elongate, contracting anteriorly. 

 The horns are small, rounded knobs rising 25 milli- 

 meters above the top of the skull from a point slightly 

 anterior to the orbits — in fact, immediately in front 

 of the preorbital malar bar; they are directed outward 

 and backward, rugose at the tips and set wide apart, 



