EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 



469 



Diplacodon theory. — The imperfectly known skull 

 of Diplacodon elatus from the base of Uinta C exhibits 

 several resemblances to B. leidyi from Chadron A, 

 and Osborn was at first disposed (1915) to regard 

 Diplacodon as containing the ancestors of Brontothe- 

 rium. (See pp. 439, 441.) 



Gregory at first regarded Diplacodon as intermediate 

 between the RJiadinorJiinus- Metarhinus group on the 

 one hand and Brontotherium on the other, but later he 

 was inclined to regard Diplacodon as possibly related 

 rather to the ancestors of the menodontine group, on 

 account of the resemblances in the premolar-molar 

 series between Diplacodon and the early members of 

 the Brontops and Menodus phyla. (See fig. 406.) 



SECTION 4. OLIGOCENE GENERA ACCEPTED AS 



VALID IN THIS MONOGRAPH 



Generic name used 

 Valid generic name and type species by Osborn in 1902 



Menodus Pomel (M. giganteus), 1849 Titanotherium. 



Megacerops Leidy (M. coloradensis), 1870- Megacerops. 



Brontotherium Marsh (B. gigas, jaw), 1873- Brontotherium. 



Brontops Marsh (B. robustus), 1887 Megacerops. 



Allops Marsh (A. serotinus), 1887 AIlops. 



Diploclonus Marsh (D. amplus), 1890 Megacerops. 



Teleodus Marsh (T. avus), 1890 Megacerops. 



Menodus. — As is fully explained on pages 204-205, 

 Pomel's name Menodus giganteus (1849) was based 

 upon the first specimen of a titanothere made known 

 to the scientific world, a fragment of a lower jaw, 

 originally described by Prout in 1847. A comparison 



Figure 400. — Restorations of lower Oligocene titanotheres of the four principal genera 

 A, Brontops rohustus; B, Menodus giganteus; C, Megacerops acen D, Brontotherium hatckeri. One-flftieth natural size. 



RJiadinorJiinus tJieory. — In 1902 Gregory observed 

 that MetarJiinus fluviatilis and RJiadinorJiinus diplo- 

 conus of the middle Eocene foreshadow Megacerops and 

 BrontotJierium in the following characters: (1) Ante- 

 rior nares very deep, nasals becoming shorter; (2) 

 bridge over infraorbital foramen very rounded; (3) 

 upward flexure of premolar series in side view and 

 "curvilinear" effect in palate view; (4) canines small 

 and sometimes swollen at base, lower canines set near 

 each other; (5) premolars relatively advanced, tetar- 

 tocones set well in toward center of crown; (6) lower 

 jaw with spoutlike incisive region (cf. R. diploconus, 

 M. earlei, M. fluviatilis). 



of a carefully drawn figure of this specimen given by 

 Leidy (1854.1, pi. 16, fig. 1) with more complete 

 material now available indicates that Menodus gigan- 

 teus is the same form as that which was later named 

 by Marsh BrontotJierium ingens. Accordingly, Meno- 

 dus giganteus Pomel has priority over Marsh's name 

 and has therefore been adopted in this work. Titano- 

 tJierium Leidy, although formerly used by the 

 present author instead of Menodus Pomel, is now 

 regarded as a synonym of that name, for reasons 

 given on pages 205-206. 



Megacerops. — -Leidy's Megacerops coloradensis was 

 founded upon a fragment of a skull (including the 



