490 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



proves that both the median incisors and the anterior 

 premolars may be shed at an early age. In other 

 words, it is demonstrable in B. dispar, as in B. irachy- 

 ceplialus, that the median incisors (i') are variable, 

 that they are found in all young animals, that they 

 may or may not drop out in later life. The lateral 

 incisors (i^) are constant. The stage of evolution 

 of the premolars in the type of B. validus is identical 

 with that in the type of B. dispar and the sagittal 

 sections of the skull in the two type specimens are 

 closely similar. The horn and nasal sections in the 

 two type specimens are essentially similar; the 

 differences (fig. 419) are due to the lateral crushing 

 and less advanced age of the skull of B. dispar. 



Chief distinctions from other species. — B. dispar is 

 thus readily distinguished from B. brachycephalus by 

 its greater measurements and more progressive 

 characters throughout; from Allops marshi by the 

 cylindrical section of its horns and by the great back- 

 ward extension of its occiput ; and from the geologically 

 succeeding B. rohvstus by the smaller size and cylindri- 

 cal section of the horns and the backward extension 

 of the occiput. 



Detailed characters oj the type of Brontops dispar. — 

 The type of Brontus dispar consists of a male jaw 

 and skull (Nat. Mus. 4941) with the occipital region 

 entirely wanting (PL XCIII). The extreme lateral 

 crushing of the skull obscures its real characters and 

 actual resemblance to the type, which Marsh sub- 

 sequently named Brontops validus. As association 

 of the skull and jaw is rare and as this specimen rep- 

 resents one of the predominant species of the middle 

 zone, the type deserves full description and measure- 

 ments in addition to those given in the comparative 

 columns above. 



Measurements of type of Brontops dispar, in millimeters 



Inferior canine (partly worn) , anterior 41 



Inferior canine, anteroposterior 30 



Length of symphysis 159 



Width of condyle - 137 



Premolars and molars: 



Teeth: Formula, 1\, P|f|. A stout lateral pair of 

 incisors persists in both upper and lower jaws, though 

 the animal is in the beginning of the eighth stage. 



The median incisors are wanting in both jaws. The 

 canines are prominent and recurved, the superior 

 canine is embraced by a posterior cingulum, while the 

 inferior canine has a faint external cingulum sur- 

 rounding all except the inner side. The anterior pre- 

 molars (pj) have dropped out in both upper and 

 lower jaws, being represented only by large alveoli; 

 p^, p^ p* have moderately developed internal and 

 vestigial external cingula and tetartocones in the 

 middle stage of development. The hypocone is a 

 prominent cingule on m'. The second lower premolar, 

 P2, is an elongate simple tooth, the metalophid not 

 being defined at all, and the hypolophid forming an 

 incipient crescent; ps, p4 present double crescents and a 

 rudimentary metastylid. The lower molars exhibit 

 partly developed external cingula, reduced opposite 

 the protoconid and hypoconid ; they also exhibit rudi- 

 ments of the metastylid. The last inferior molar ip 

 highly characteristic of the Brontops- Menodus group, 

 a broad internal shelf extending inward from the 

 hypoconulid, with a trace of a posterior cingulum 

 beneath it. The much crushed jaw exhibits a shallow 

 symphysis, a large mental foramen beneath pa. 

 Plate XCIV gives its partly restored outlines. 



Skull: The skull exhibits the short, truncate, and 

 cleft nasals, short horns with a subtrihedral section 

 at the base, rounded malar, nasal and connecting 

 ridges, and roimded faces. The posterior face is 

 broad and imiformly convex, as in the other advanced 

 members of this phylum. The horns have a charac- 

 teristic angle of the outer border, leadiag into a 

 rounded summit, as shown in the section. The 

 zygomatic arches are robust and convex, but they are 

 too much crushed for an accurate section. 



Type of Brontops validus. — The type of the syno- 

 nymic species Brontops validus (Nat. Mus. 4290) is a 

 male in the seventh growth stage. It is well figured 

 in Plates LXXXVIII, LXXXIX of the present mono- 

 graph. The principal measurements are as follows: 



Measurements of type of Brontops validus 



Millimeters 



Molar-premolar series 320 



Condyles to premaxillary tips 660 



Free portion of nasals, length 85 



Free portion of nasals, breadth 120 



Length of horn to nares 199 



There are two incisors above and below ; the canines 

 are large and pointed, indicating that this specimen 

 was a male; the fourth premolar has a rather feeble 

 tetartocone without a distinct valley separating it 

 from the protocone; the hypocone of the third su- 

 perior molar is a shallow ridge arising from the cin- 

 gulum. The species is clearly characterized by the 

 short, square nasals and horns borne well forward; 

 but it can not be separated specifically from the pre- 

 viously described Brontops dispar. 



General characters of B. dispar. — The measure- 

 ments of the skulls referred to B. dispar are singularly 



