EVOLUTION OF THE SKULL AKD DENTITION OF OLIGOCENE TITANOTHERES 



571 



slopes into a broad saddle shape, more uniform in 

 width anteroposteriorly than in B. rohustus, partly 

 owing to lateral crushing, but the vertex is much 

 longer, especially as seen in the backward extension 

 of the occiput behind the zygomata. 



The parietal vertex is very wide, with overhanging 

 supraorbital and supratemporal crests, terminating 

 in a powerful rugose occiput which is broader and 

 more shelf-like superiorly than in M. rohustus. The 

 buccal processes are flatter superiorly, though the 

 area of the section is not greater than in the large 

 specimen Am. Mus. 6346, which is provisionally re- 

 ferred to B. curtum; as in B. leidyi there is a flange 

 extending inward from the center of the zygoma 

 (fig. 470, c). The pillars of the occiput are very 

 powerful, with a pair of tuberosities (fig. 378, F, k, k) 

 projecting upward on either side of the median line. 

 We have seen these tuberosities in Megacerops and in 

 prophetic form in B. leidyi. They represent the 

 development of special muscular fasciculi or tendons 

 for the semispinalis capitis muscles and the liga- 

 mentum nuchae; these tuberosities are rudimentary 

 or absent in certain skulls of Menodus and are re- 

 placed by two pits in Megacerops acer. There is a 

 wide union between the post-tympanic and post- 

 glenoid processes; each pterygo-alisphenoid wing is 

 cleft on its extreme lower border for the insertion of the 

 pterygoid muscle; the pterygoid wings are somewhat 

 shorter than in Brontops rohustus. As in B. leidyi 

 and in contrast to B. rohustus, there are no paired 

 protuberances at the junction of the basioccipital 

 and basisphenoid for the attachment of the recti 

 capitis antici majores. The vomerine ridge is reduced. 



Juvenile stage of growth. — A relatively young male 

 skull in the fifth stage of growth (Nat. Mus. 4262), 

 with which are associated the backbone and limbs 

 (see above), illustrates the form of the horns, nasals, 

 and zygomata of this species in the young condition 

 and serves to strengthen the identification of Titanops 

 elatus with B. gigas. It was found at Hat Creek, 

 Wyoming, and is recorded by Hatcher as from the 

 lowest level of the upper Titanoiherium zone (Chadron 

 C). The horns measure 275 millimeters. The for- 

 mula is If, Pf; the upper incisors are small and 

 conic without posterior cingulum; the lower incisors, 

 of which the outer one is preserved, have a crenulate 

 posterior cingulum; the superior canines measure 33 

 millimeters and are short and obtuse, with a charac- 

 teristic posterior slope and cingulum; the inferior 

 canines have a postero-internal, less prominent 

 cingulum. The tetartocones on p^-p* are very dis- 

 tinct; p* has a mesostyle as in the type specimen. 

 The jaw resembles that of the type of B. gigas in the 

 chin and angle; the chin is extremely shallow. 



Character of the female slculls. — The determination 

 of a small skuU (Am. Mus. 1006) as a female of this 

 species is rendered probable by its discovery on the 



same level as the large B. gigas (Am. Mus. 492), as 

 well as by the sections of the nasals, horns, and zygo- 

 mata. The disparity in size between this female 

 skull and the male skull is very marked. The animal 

 is well advanced in the seventh stage of growth. 

 The deep connecting crest and shape of the nasals 

 resemble those of B. gigas, but the horns are very 

 short and obtuse, and the buccal processes are much 

 less robust. The malar bridge is broader than in 

 the male skull, as in B. curtum female, and the median 

 ridge is obtuse and low. The post-tympanic has a 

 characteristic broad, flattened union with the post- 

 glenoid; the palatine aspect also resembles that of 

 B. gigas on a small scale, with a smooth basisphenoid. 



Sexual characters are observed not only in the 

 extremely small canines but apparently also in the 

 reduction of the incisors, so far as we can judge from 

 the alveoli and fangs. Careful examination reveals 

 the reduced fang of a lateral incisor on the right side. 

 The canines differ from those of the male not only in 

 the much smaller size but also in lacking the swollen 

 appearance; they are, in fact, recurved, conic. The 

 premolars differ from those of the male in the lesser 

 size of the tetartocones and in the pronounced non- 

 crenulate cingulum. 



A very large skull in the National Museum (No. 

 4244), in the fourth stage of growth, either a large 

 female or a juvenile male with imperfectly developed 

 horns, is recorded by Hatcher from the top level of 

 the upper zone. The horns measure 196 millimeters, 

 with the anterior ridge more prominent than in the 

 type. Other characters conform closely to the type — 

 namely, I^, distinct tetartocones on p*, m^ with a small 

 but distinct hypocone, buccal processes flattened. A 

 variation is the loss of p'. 



Female characters in general. — A comparison of the 

 supposed female skulls of Brontotherium gigas and of 

 B. curtum shows that since the male progressive 

 characters are largely correlated with excessive size 

 and horn development the females actually appear 

 to be more primitive — namely, the horns are shorter, 

 the zygomata are slighter, the occiput is less extended 

 backward, the malar bridge is wider, the orbit is 

 larger, the external auditory meatus is somewhat more 

 open. 



The apparent loss or reduction of the incisors, the 

 somewhat retarded condition of the tetartocones, and 

 the pronounced development of the internal and 

 external cingula are, however, verj^ difficult facts to 

 explain as sexual characters. 



Additional observations on the supposed female of 

 Brontotherium gigas. — A female skull in the American 

 Museum (No. 1006) which has been referred to this 

 species presents a very puzzling specimen. The sections 

 of the horns and nasals offer resemblances and differ- 

 ences to those of the type of B. (Titanops) elatum which 

 have been assumed to indicate a female of this species. 



