EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 



387 



In other words, while the ratio of breadth to length 

 falls from 59 to 45 per cent, the ratio of the length of 

 the face to the entire length of the cranium, or facio- 

 cephalic index, remains at 48. This is a very impor- 

 tant distinction, because progressive abbreviation of 

 the face is characteristic of all the true Oligocene 

 titanotheres correlated with dolichocephaly but is not 

 found in these dolichorines. We observe other corre- 

 lated dolichocephalic changes in the skull — namely, 

 (1) the occipital condyles are set broadly apart; (2) 

 the external auditory meatus becomes widely open; 



(3) wide spaces arise between the cranial foramina; 



(4) the horn rudiments rise chiefly on the nasals and less 

 On the frontals; (5) the nasals have a long exposure on 

 the top of the skull; (6) there are correlated changes 

 in the teeth. 



The teeth also show the following correlations with 

 doliochocephaly : (1) The opposite molar-premolar 

 series become parallel; (2) the palate is narrowed, 

 elongated, and arched; (3) the individual molar teeth 

 are elongated or laterally compressed; (4) the pre- 

 maxillary and mandibular symphyses become greatly 

 elongated; (5) the jaws become long and slender, and 

 there is an increasing distance between ms and the 

 posterior border of the jaw, the coronoid becoming 

 relatively low and backwardly recurved, its anterior 

 edge oblique rather than transverse, and the chin 

 shallow and sloping (hence the term hyognaihus, or 

 hog-jawed, applied by Scott and Osborn); (6) simi- 

 larly the inferior molars become elongate; (7) the 

 whole upper tooth row shifts forward with reference 

 to the orbit. 



The recently discovered EometarJiinus is described 

 on pages 200, 419, 420. 



Mesatirhinus Osborn 



Plates XVII, L, LXXI, LXXII; text figures 26, 27, 33, 122, 211, 

 217, 219, 265, 302, 327-331, 333, 339-341, 483, 511-514, 516, 

 520-523, 526, 558, 559, 562-571, 586, 647, 656, 661, 686, 702, 

 713, 716, 724, 745 



[For original description and type references see p. 182. For slieletal eliaracters 

 see p. 636] 



Geologic horizon. — Bridger C and D and Washakie A. 



Generic characters. — Middle Eocene titanotheres of 

 small but increasing size; basilar length, 354-485 

 millimeters. Mesaticephaly progressing to dolicho- 

 cephaly. Horns incipient, borne chiefly on the nasals; 

 prominent infraorbital malar shelf; nasals elongate, 

 laterally recurved; cranium profile convex; face de- 

 flected; sagittal crest gradually broadening; no sec- 



ondary palate. Humerus short; tibia relatively long; 

 carpus and tarsus narrow; astragalus with elongate 

 neck; metapodials relatively elongate. 



Geologic distribution. — There is the Eometarhinus of 

 Huerfano B (Bridger A), and the Mesatirhinus Junius 

 of Bridger B. In Bridger C and D, also in Washakie 

 A, there first appears a rich array of small titanotheres, 

 which are readily distinguished from the contemporary 

 species of Palaeosyops, Telmatherium, and Manteoceras 

 by the generic characters enumerated above. These 

 animals are related on the one side to Metarhinus and 

 on the other, by progressive changes, to Dolichorhinus, 

 and the phylum is therefore regarded as central. The 

 phylum is divided into the smafler, more primitive 

 species Mesatirhinus megarhinus and the partly con- 

 temporaneous, more progressive species M. petersoni. 

 These species are contemporaneous in Bridger C 5, and 

 both animals are found in Washakie A, which is 

 evidence that they are contemporaneous and not 

 successive species. At the summit of Uinta B 1 occurs 

 the larger and more progressive "Mesatirhinus" 

 superior, with partly flattened cranium. This animal 

 is here referred to Dolichorhinus. 



In the Uinta region Metarhinus is so abundant in the 

 fluviatile sandstones of Uinta B 1 that the horizon is 

 named the Metarhinus zone. The animals disappear 

 at the summit of this zone in the " Metarhinus sand- 

 stones." 



The synopsis of these species is as follows : 



Mesatirhinus Junius (Leidy)?, Bridger B, a diminu- 

 tive animal. (See fig. 325.) 



Mesatirhinus megarhinus (Earle), Bridger C and 

 Washakie A. Skull small (about 354 by 170 mm.); 

 cephalic index, about 53; faciocephalic index, 48; 

 palatal crests narrow; nasofrontal horns incipient; 

 premolars in less advanced stage. 



Mesatirhinus petersoni Osborn, Bridger C and D and 

 Washakie A. Skull of intermediate size (about 425 by 

 205 mm.); cephalic index, 49; parietal crest narrow; 

 faciocephalic index, 48; premolars in more advanced 



Mesatirhinus (Dolichorhinus) superior Eiggs, sum- 

 mit of Uinta B 1. Skull larger (485 by 240 mm.); 

 cephalic index, 52; faciocephalic index, 48; parietal 

 crest spreading; cranium flattened on top. 



It is important to note that although these three 

 species succeed each other progressively and this pro- 

 gression leads directly to Dolichorhinus, there is no 

 proof of direct phyletic succession. 



