THEORIES AS TO ORIGIN, ANCESTRY, AND ADAPTIVE RADIATION 



787 



Brachycephaly, extreme of, in the Palaeosyopinae. 



Dolichocephaly, extreme of, in the Dolichorhininae. 



Cyptocephaly, deflected facial region, extreme of, 

 in Dolichorhininae. 



Cyptocephaly, upturned facial region, beginnings of, 

 in Metarhinus, Rhadinorhinus (possibly prophetic of 

 Megacerops). 



The pi-ofound changes in the cranial axis produced 

 by the cyptocephaly of JDolichorJiinus in comparison 

 with the orthocephaly of Palaeosyops are shown in the 

 accompanying diagrams of Palaeosyops and Dolicho- 

 rhinus. Gregory was the first to observe the up- 

 turned facial region of Metarhinus as possibly sugges- 

 tive of the greatly abbreviated and upturned muzzle 

 of Megacerops of the lower Oligocene. 



The deflected cranial axis of Dolichorhinus, as 

 described in Chapter V, section 4, is connected 

 functionally with its peculiar habits of feeding. These 

 cranial sections are important not only as illustrating 

 the flexure in the cranial axes, but also the great 



development of the air sinuses of the facial and cranial 



regions and the relatively small area occupied by the 



brain. 



ZYGOMATIC INDICES 



As explained in Chapter V, section 1, the zygomatic 

 index is obtained as follows: Transverse measure- 

 ment of the zygomatic arches X 100 h- basilar length 

 of the skull. 



The terms brachycephaly and dolichocephaly as 

 used in this monograph are based chiefly on the 

 zygomatic index in the comparison of the total length 

 of the skull with the width across the zygomatic 

 arches, rather than in the comparison of the actual 

 width and length of the cranium proper. From the 

 following table of zygomatic indices it appears that 

 the primitive titanotheres, like other primitive peris- 

 sodactyls, are dolichocephalic or mesaticephalic. 

 Arranged in descending order according to increase of 

 zygomatic breadth, irrespective of direct descent, the 

 crania are as follows : 



Proportions of length and hreadth of crania of titanotheres 



[Measm-ements in Diillimeters] 



Form of skull 



I. Primitive crania: 



Phenacodus wortmani 



Eohippus venticolus 



Hyrachy us sp 



Lophiodon leptorhynclius 



Eotitanops borealis 



II. Progressive dolichocephaly: 



Metarhinus earlei 



Mesatirhinus petersoni (No. 1556), 

 Dolichorhinus (No. 1852, ?) 



III. Progressive brachycephaly: 



Manteoceras inanteoceras — 



No. 12678 



No. 2353 



No. 1569 ■ 



Telmatherium ultimum 



IV. Palaeosyops series: 



Palaeosypos leidyi (No. 1544) 



V. Brontops series, progressive brachycephaly: 



Brontops braohy cephalus 



Brontops dispar 



Brontops robustus 



Diploclonus amplus 



VI. Brontotherium series, progressive brachycephaly: 



Brontotherium leidyi 



Brontotherium hatcheri 



Brontotherium gigas 



Brontotherium platyceras 



VII. Menodus series, dolichocephaly, progressive dolichocephaly: 



AUops serotinus 



Allops crassicornis 



Menodus giganteus 



147 

 129 

 263 

 360 

 313 



393 

 438 

 550 



500 



"465 



492 



500 



415 



58 



53 



122 



176 



163 



240 

 207 

 240 



°294 

 277 



" 310 

 300 



310 



73-87 



77-87 



91 



66 



74 



89 



110 



74 



75 



62-70 



Dolichocephalic. 

 Do. 

 Do. 

 Do. 

 Do. 



Mesaticephalic . 

 Dolichocephalic. 

 DoUchocephalic (hy- 

 perdolichocephalic) 



Mesaticephalic. 

 Do. 

 Do. 

 Do. 



Brachycephalic. 



Do. 

 Do. 

 Do. 

 Do. 



DolichocephaUc. 

 Mesaticephalic. 

 Brachycephalic. 

 Hyperbrachycephalic. 



Mesaticephalic . 



Do. 

 Dolichocephalic . 



' Estimated. 



101959— 29— VOL 2- 



