EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 



565 



founded on a skull (Nat. Mus. 1216), while B. gigas 

 rests upon a lower jaw. The type skull has the nasals 

 longer and the horns shorter and less flattened than 

 in the type of Titanops elatus, and the same is true of 

 the other three skulls which stand nearest to the type 

 (Univ. Wyo. Mus. 1, Field Mus. P 5926, Am. Mus. 

 1070). The range in the principal measurements of 

 these skulls as compared with B. gigas, the succeeding 

 stage, is as follows: 



Range in measurements of Brontotherium gigas elatum and B. 

 hatcheri (gigas?), in millimeters 



pi-m' 



Pi-p4 



Mi-m3 



Pmx to condyles 

 Zygomatic index 



Nasal length 



Horn length 



348-353 

 126-133 

 218-241 

 770-830 

 82-87 

 70-87 

 275-365 



« 310-320 

 118-132? 



192-? 



7 10-? 



85?-105 

 200-265? 



Skull Am. Mus. 1070 has the horns a little longer 

 and more slender than in the type of B. hatcheri, but 

 its nasals are as short as in the typical B. gigas elatum 

 and its premolars are as long as in that form. Hence 

 it appears to connect these two stages. 



Comparative measurements of jaws of B. hatcheri and B. gigas, 

 in millimeters 



Symphysis to angle 



Condyle to symphysis. _. 

 Depth, condyle to bot- 

 tom of angle 



Depth of jaw below mj-. 

 Molar-premolar series-.. 



Premolars 



Molars 



M3, transverse 



Mj, longitudinal 



Canine enamel, anterior. 

 Canine enamel, antero- 

 posterior 



Length of symphysis 



B.hatcberi, 



Am. Mus. 



1070 



605 

 600 



300 

 100 

 375 

 ■120 

 263 



48 

 103 



35 



29 



B. gigas, 

 Yale Mus. 

 12009 (type) 



620 

 634 



305 

 105 

 365 

 117 

 250 



47 

 118 



34 



30 

 173 



B. gigas 



(hatcheri?), 



Nat. Mus. 



4262 



623 



" Titanops 



elatus," 

 Yale Mus. 

 12061 (type) 



" Estimated. 



The lower molar grinding teeth of B. hatcheri 

 (263 mm.) exceed in linear dimensions those of either 

 the B. gigas type specimen (250) or those of the 

 T. elatus type specimen (260); consequently the pro- 

 gressive characters of B. gigas elatum are to be seen 

 in the elongation of the horns, the elevation of the 

 connecting crest, and the abbreviation of the nasals 

 (figs. 465, 470), rather than in the increasing length 

 of the grinding series. 



The jaws of B. hatcheri, B. gigas, and B. elatum 

 (figs. 468, 471) are influenced so much by age, sex, 

 and individual variation that it is diQacult to separate 

 them specifically. 



Brontotherium tichoceras (Scott and Osborn) 



{Menodus tichoceras Scott and Osborn, 1887; " Megacerops 

 tichoceras" Osborn, 1902) 



Plate CLXXXVII; text figures 177, 469 



[For original description and type references see p. 219] 



Geologic horizon. — Titanotherium zone of South 

 Dakota. Level unrecorded. 



Specific and generic characters. — Skull about as large 

 as in B. gigas. I^, P^. Superior canines and both 

 pairs of incisors well developed ; grinding teeth without 

 cingula, premolars with large, distinct tetartocones; 

 nasals of medium length and thickness, contracting 

 anteriorly; base of horns of stout, obliquely transverse 

 section. Buccal processes very prominent, slightly 

 concave superiorly; postglenoid and post-tympanic 

 widely conjoined. Grinding series 330 millimeters — 

 that is, about the same as in smaller skulls of B. gigas. 



Materials. — The species is known only from the 

 type, in the Museum of Comparative Zoology, Har- 

 vard University, a skull which lacks the horns. 



The absence of the summits of the horns and of the 

 occiput and the unfortunate loss of the cutting teeth 

 (which has occurred since the original description of 

 this skull was published) prevent us from placing 

 this species phyletically wit i certainty. It assuredly 

 does not belong to Menodus but is rather a member of 

 the brontotheriine group, as shown by the large 

 rounded tetartocones and midparietal convexity. It 

 resembles Megacerops in the general aspect of the non- 

 cingulate grinders, the smooth basisphenoid, and some- 

 what also in the basal horn section and the midparietal 

 protuberance and convexity; it radically difl^ers from 

 this genus in the much greater development of the 

 cutting teeth, or incisors, and the wide separation of 

 the canines. It resembles Brontotherium in the form 

 and size of the cutting teeth (so far as the characters 

 of these parts can be judged from the original figures), 

 the wide union of the postglenoid and post-tympanic 

 processes^ and the narrowing of the nasals anteriorly. 

 On the whole, therefore, B. tichoceras appears to stand 

 closest to the Brontotherium phylum, although it lacks 

 especially the characteristic transverse basal horn 

 section, which seems to be merely an exaggeration of 

 the basal section of the type of B. hatcheri. If the 

 section were preserved at a little higher point on 

 the horn it might show some of the characteristic 

 flattening. 



Additional ohservations on the type. — A reexamination 

 of the type in 1902 afforded opportunity for additional 

 measurements and sections of the horns and zygomatic 

 arches. The canines in the type, although now lost, 

 were large and indicate that this was probably a male 



