EVOLUTION OF THE SKULL AND TEETH OP EOCENE TITANOTHERES 



259 



compressed laterally or stretched out longitudinally as 

 if composed of india rubber, all parts being elongated 

 alike. (See fig. 217.) This appearance is entirely 

 deceptive, because every bony element of the skull 

 has a different rate of elongation. As above noted, 

 although the face of the titanothere is abbreviated 

 (brachyopic), the cranium is elongated (dolichocranic), 

 and it is chiefly the midregion of the cranium that is 

 notably elongated — that is, the region between the 

 orbits and the postglenoid processes. There is no 

 predetermined innate or invariable correlation of 

 brachycephaly or dolichocephaly in all parts of the 

 skull, nor is there any fixed correlation between elonga- 

 tion or abbreviation of the skull and of the limbs 

 respectively. (See law of correlation, Chap. XI.) 



ZYGOMATIC-CEPHAHC INDICES IN THE TITANOTHERES 

 AND OTHER PERISSODACTYLS 



RELATIVE VALUE OF INDICES 



The proportions and indices applied to different 

 groups of mammals are largely relative. The terms 

 applied to the human cranium and the indices are as 

 follows : 



Brachycephalic = brachycranial, 80.1-100 

 Mesaticephalic == mesaticranial, 75. 1-80 

 Dolichocephalic = dolichocranial 60-75 



Among the perissodactyl ungulates that have ordinal 

 relationships to the titanotheres some of the indices of 

 total length and width of skull are shown in the 

 accompanying table. 



Indices oj length of sTcuU of perissodactyl ungulates related to the titanotheres 



Sumatran rhinoceros (Rhinoceros (Dicerorhinus) sumatrensis) . 



Black African rhinoceros (R. (Opsiceros) bicornis) 



South American tapir (Tapirus terreStris) 



White African rhinoceros (R. (Ceratotherium) simum) 



Indian tapir (Tapirus indicus) 



Domestic horse (Equus caballus) 



Domestic horse (Equus caballus) 



Domestic ass (Equus asinus) 



Basilar length, 



premaxillaries 



to condyles 



(millimeters) 



580 

 568 

 355 



387 

 513 



Transverse 



width across 



zygomatic arches 



(millimeters) 



345 

 320 



178 



183 

 202 



59 



56 



50 



43-50 



47 



39 



40. 4-44 1 



46. 9-49. 9 



Most of the skulls of the above-named species are dolichocephalic in comparison with the skulls of titanotheres. 

 A standard of skuU proportions among the perissodactyl ungulates, including the rhinoceroses, tapirs, horses, 

 and titanotheres, may be established as shown in the table below. 



Summary of the zygomacephalic indices of the perissodactyls 



INDICES OF SKULLS OF EOCENE AND 

 TITANOTHERES 



OLIGOCENE 



The study of these proportions and indices of the 

 titanotheres demonstrates that the skull in this 

 family presents an ascending scale from primitive 



dolichocephalic ancestors like Lambdotherium and 

 Eotitanops, which, on the one hand, evolved into 

 extremely broad-headed forms like Palaeosyops and 

 Megacerops, and, on the other, into extremely long- 

 headed forms like Mesatirhinus and Dolichorhinus, 

 as shown in the following table: 



