570 



TITANOTHERES OP ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



Teeth: The incisors are represented by two pairs of 

 rather small alveoli with a slight diastema between; 

 the lateral alveolus is slightly larger than the median; 

 the canines are low-crowned, obtuse, flattened, and 

 cingulate on the postero-internal surface and probably 

 indicate a male; there is a narrow diastema (11 mm.) 

 between the canines and second premolars and no in- 

 dication of pi. The premolars exhibit no external or 

 internal cingula; there is an external crenulation only 

 on the valleys of the molars ; the grinding teeth there- 

 fore are decidedly noncingulate. M2 and ms exhibit 

 a rudimentary metastylid; ms has a broad-cupped hy- 

 poconulid with a slightly crenulate internal crest. 



The presence of lower incisors and the shape of the 

 canines differentiate this jaw from that of the animal 

 later described by Cope as Symhorodon tonus. The 

 looped structure of the hypoconulid of ma is different 

 from that in the Menodus series. The closest resem- 

 blances in the jaw, canines, absence of cingula, and in 

 ms are to the animal later termed by Marsh Titanops 

 elatus. 



Ohservations on the type skull oj B. {Titanops) 

 elatum considered as pertaining to B. gigas. — The 

 cranial vertex is somewhat crushed and moderately 

 broad, with a rugose crest overhanging the orbital and 

 temporal fossae; the nasals are rather narrow, of me- 

 dium length; the external auditory meatus is com- 

 pletely coalesced below, the angle of the jaw is 

 slightly depressed, and also extended backward. 

 The age of the skull is determined as in the ninth 

 stage, all the internal cusps of the grinding teeth 

 being worn except upon the last superior molar. 



It should be noted that the zygomatic arches, the 

 premaxillaries, and the anterior portion of the jaw 

 of this skull are wanting. 



Teeth: The superior premolars exhibit a distinct 

 internal cone, the tetartocone, which is well marked 

 off and separate from the deuterocone on p', p*. 

 P* also apparently exhibits a mesostyle. The hypo- 

 cone is fairly prominent but partly connected with 

 the cingulum on m^ The internal cingulum of the 

 superior premolars is sessile and crenulate. The 

 lower premolars and molars exhibit a vestigial or 

 basal cingulum except on nis, in which the cingulum, 

 as in the type of B. gigas, is slightly more decided. 

 Ms exhibits a broad hypoconulid and crenulate 

 internal crest, a characteristic phyletic character. 



Measurements of the type jaw are given above. 



The type skull is unfortunately incomplete, so that 

 the chief dental measurements are lacking, but the 

 basilar length and the dimensions of the nasals and 

 horns are approached by a finely preserved skull in 

 the American Museum (No. 492). This is remark- 

 able for its very large true molars (241 mm.), its 

 great basal length (830 mm.), and its high zygomatic 

 index (87). The horns are less flattened than in 

 B. medium and B. curtum but more flattened than in 

 B. hatcJieri. 



Two other skulls provisionally referred to this 

 stage (Carnegie Mus. 341, Nat. Mus. 4262) agree 

 closely in measurements with each other but have 

 the true molars (218 mm.) and the horns (275 mm.) 

 considerably shorter than in Am. Mus. 492; another 

 skull (Nat. Mus. 4244) has much shorter horns 

 (170 mm.). Thus these skulls seem to connect 

 B. gigas elatum with B. Tiatcheri and serve to illus- 

 trate the variability in length of the horns and of the 

 nasals. 



The female skull (Am. Mus. 1006) which is referred 

 to this species approaches the male skulls in the 

 dimensions of the cheek teeth as well as in the sec- 

 tions of the horns and nasals. But it differs from all 

 typical BrontotJierium skulls in having well-defined 

 internal cingula and retarded tetartocones in the 

 premolars; so that relationship with Brontops rohustus 

 might be suspected were it not for the much closer 

 agreement in dental measurements with B. gigas. 



Fine male slcuU (Am. Mus. 492). — The fully adult 

 characters of the males are admirably shown in the 

 nearly perfect American Museum skull No. 492, 

 which is in the seventh stage of growth. Two smooth, 

 rounded incisors are preserved on one side above; the 

 canines measure 35 millimeters anteriorly as compared 

 with 41 in B. robustus; they are shorter and obtuse 

 (Pis. XX, B, CLXXIII, CLXXIV). There is no 

 diastema behind the canine. The greatly reduced 

 first premolar is pressed close to the canine and tends 

 to drop out; the premolars can readily be distinguished 

 from those of B. rohustus; the tetartocones although 

 low and obtuse are about two-thirds as large as the 

 deuterocones and quite distinct throughout. The 

 premolar cingula, however, have retrogressed as in 

 B. leidyi, being practically vestigial on the outer side 

 and less distinct on the inner side than in B. ro- 

 hustus; the molar cingula also are nearly obsolete. 

 On m' is a sessile hypocone connected with the 

 cingulum. 



The grinding teeth, considered so important to the 

 welfare of large herbivorous quadrupeds such as this, 

 are in certain respects in a condition of retrogression. 

 In the type of B. gigas the bluntness and crenulation 

 of the internal cingula precede degeneration. In 

 this very large skull (Am. Mus. 492) they measure 

 353 millimeters, only 3 millimeters more than in the 

 type skull of B. rohustus. This contrast between the 

 arrested development of the teeth and the pro- 

 nounced evolution of the dominant protuberances of 

 the skull has its parallel among the Dinocerata. 



The inverse relations of the nasals and horns illus- 

 trate the law of compensation of growth, the free 

 nasals being here absorbed and reduced in compen- 

 sation for the great elongation and expansion of the 

 horns, which are now strengthened by a very promi- 

 nent crest, attaining a vertical thickness of 140 milli- 

 meters as compared with 78, the maximum thick- 

 ness in B. rohustus. Behind the horns the skull 



