468 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



canines and of the grinding teeth but differ in the 

 marked degeneration of the incisor teeth. In the 

 loss of incisors they parallel the Megaceropinae. 

 In contrast to the Brontops series the limbs and 

 arches are slender, the bones of the girdles.are relatively 

 high and narrow, and the feet are of the elongate, 

 dolichopodal type. Thus the menodonts are relatively 

 cursorial in contrast with the typically graviportal 

 brontopines. 



Brontotheriinae. — The gigantic titanotheres of the 

 subfamily Brontotheriinae appear to be related 

 either to the Diplacodon or to the Ehadinorhinus 



with the elongation of the horns. The cheek teeth 

 are less elongate than in Menodus and have more 

 rounded crowns and nearly obsolete cingula, well 

 fitted for cutting and crushing coarse vegetation. 



Megaceropinae. — The Megaceropinae are much 

 smaller animals than the brontotheres, distinguished 

 by skulls of intermediate proportions, with tall, rather 

 slender and cylindrical horns placed well forward 

 above the eyes. The incisors are absent. It appears 

 probable that the snout was narrow and terminated in 

 a pointed Up like that of the black rhinoceros of Africa. 

 The face was very short, narrow, and upturned, bring- 



MENODUS 



MEGACEROPS 



BRONTOTHERIUM 



bnuJiycefhalus 



'brndiyc^Jtalus 



MtngoruKems 



FiGUKE 399. — Sections at base of horn in five principal lower Oligocene phyla of titanotheres, arrang 



according to ascending geologic levels 

 Drawn to the same scale. 



phylum of the upper Eocene.^^ They are distinguished 

 by the precocious development of the horns, the rapid 

 transformation of the premolar teeth, the stout, 

 obtuse canines, the retention of two pairs of upper 

 and lower incisor teeth that have cingulate crowns in 

 contrast to the smooth, rounded crowns in the Bron- 

 topinae. The horns early acquire at their extremities 

 a transversely oval shape, which finally extends down 

 to the base of the horn. The cranial vertex is ex- 

 tremely long and narrow, but the great buccal proc- 

 esses at the sides of the head develop pari passu 



" See pp. 434, 441, 469, 560. 



ing the mouth almost up to the level of the eye. The 

 small obtuse canines were brought close together 

 toward the median line. The cusps and cutting edges 

 of the cheek teeth were even more rounded than in 

 Brontotherium and entirely devoid of cingulum. 



POSSIBLE EOCENE ANCESTORS OF THE BEONTOTHEEIINE 

 GROUP 



We may again consider the evidence and theories as 

 to the transitions between Eocene and Oligocene 

 titanotheres. (Compare Diplacodon, Rhadinorhinus, 

 pp. 439, 441, 470-474.) 



