384 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



its diflerences from the typical Palaeosyops. This is 

 now known as MesatirMnus megarhinus. 



1894. Peterson explores horizon B 2 of the Uinta 

 Basin and discovers a remarkable long-skulled form. 



1895. This long-skulled form is described by Osborn 

 as " Telmatotherium cornutum," which is now known 

 to be a synonym of DolocJiorMnus hyognatJius. 



1895. Osborn also describes a smaller form from 

 Uinta B 1 as "Telmatotherium diploconum." This is 

 now known as Rhadinorhinus diploconus. 



1895. Osborn also describes, from Uinta B 2, SpJieno. 

 coelus uintensis, a form that still remains problematical. 



1895. Earle soon afterward points out the ances- 

 tral relationship of "Palaeosyops megarhinus" to " Tel- 

 matotherium cornutum," an affinity now recognized as 

 that of Mesatirhinus to Dolichorhinus. 



1895. Hatcher recognizes " Telmatotherium cor- 

 nutum" as a new genus, namely, Dolichorhinus, pos- 

 sessing horns, but not directly ancestral to any of the 

 Oligocene titanotheres. 



1894-1906. American Museum explorers in the 

 Bridger and Washakie Basins, under Peterson, Mat- 

 thew, and Granger, bring together good material of 

 the "Palaeosyops megarhinus" type. 



1908. Osborn reviews the narrow-skulled or doli- 

 chorhine Eocene titanotheres . with the following 

 principal results: 



{a) Hatcher's term DolicTiorhinus is adopted, and 

 D. cornutus is shown to be a synonym of D. hyognathus, 

 both occurring in Washakie B. A new species, Doli- 

 chorhinus intermedius, is described. 



(h) Earle's Palaeosyops megarhinus from Bridger B 

 and Washakie A is made by Osborn the type of the 

 new genus " Mesatirhinus," ancestral to Dolichorhinus. 

 The new species Mesatirhinus petersoni is described 

 from Bridger C. 



(c) A related group, including small Eocene tita- 

 notheres with slender limbs and relatively short, 

 narrow skulls, from Uinta B 1 and Washakie B, is 

 recognized by Osborn as the distinct genus Metarhinus, 

 including M. fluviatilis, M. earlei, and [?] Telmatothe- 

 rium diploconum. 



(d) The opinion is expressed that Metarhinus and 

 Dolichorhinus represent the long-skulled form of the 

 same stock that gave rise to the relatively broad- 

 skulled Manteoceras. 



1909. Douglas describes two new species of Doli- 

 chorhinus {D. heterodon, D. longiceps) from Uinta B 2. 



1912. Riggs greatly extends our knowledge of the 

 dolichorhines of Uinta B 1 and revises and expands 

 the species Metarhinus, Mesatirhinus, and DolicJio- 

 rhinus, establishing the new subfamily Dolichorhi- 

 ninae and basing the new genus Rhadinorhinus on the 

 type R. abbotti, including also the " Telmatotherium dip- 

 loconum" of Osborn. 



1919. Osborn describes Eometarhinus from the 

 upper part of the Huerfano formation, representing an 

 extremely primitive ancestor of Metarhinus. 



The original and the present determination of the 

 synonymy of these species is thus as follows: 



Palaeosyops vallidens = Dolichorhinus vallidens. 

 Palaeosyop.g hyognathus = Dolichorhinus hyognathus. 

 Palaeosyops megarhinus = Mesatirhinus megarhinus. 

 Telmatotherium cornutum = Dolichorhinus hyognathus. 

 Telmatotherium diploconum = Rhadinorhinus diploconus. 



COMPARISON WITH MANTEOCERAS 



There are in Manteoceras, Mesatirhinus, and Doli- 

 chorhinus resemblances which prove that these animals 

 sprang from the same stock. They appear specially in 

 the comparison of the skulls of M. manteoceras and 

 Mesatirhinus megarhinus; in other words, the ancestral 

 and atavistic characters of Mesatirhinus are those 

 which it has in common with Manteoceras, among 

 which are (1) preorbital concavities; (2) nasals long, 

 decurved, truncate distally; (3) posterior nares com- 

 pressed, or narrow space between the palatines and 

 pterygoid plates; (4) zygomatic arches shallow; (5) 

 occiput broadly depressed; (6) pit in the parietal 

 vertex of the Manteoceras skull represented by a long 

 slit in the Mesatirhinus skull; (7) angulation of the 

 malars of Manteoceras represented by the suborbital 

 shelf of Mesatirhinus. Their ancestral affinity to 

 Manteoceras is also seen in (8) the position of the 

 horns above the preorbital concavities; (9) the 

 elongate form of the horn rudiments. There is a 

 decided departure from the position of the horn rudi- 

 ments of Manteoceras (PI. XVII) — namely, in that in 

 the Dolichorhininae the horn swelling is chiefly a pro- 

 tuberance of the nasal bones, whereas in Manteoceras 

 the horn swellings are chiefly on the frontal bones, the 

 nasofrontal suture of the dolichorhines being pushed 

 back by the remarkable elongation of the nasals. (10) 

 Another distinction is that in Manteoceras the horn 

 swelling is decidedly in front of the orbit, whereas in 

 Mesatirhinus it lies more directly above the orbit. 

 Other differences appear in connection with the fact 

 (11) that the face is relatively longer in the dolicho- 

 rhines than in Manteoceras. 



The face is relatively longer than in Manteoceras. , 

 Correlated with this is the fact that in Metarhinus, 

 Mesatirhinus, and Dolichorhinus the grinders are 

 farther forward with reference to the orbit than in 

 Manteoceras — that is, in the members of these groups 

 the postorbital process of the malar lies above the 

 mesostyle of m^, and in Manteoceras it lies above the 

 parastyle of m^ Similarly in Dolichorhinus the 

 lacrimal lies above the mesostyle of m^, in Manteoceras 

 above the mesostyle of m'. In Dolichorhinus this 

 relation appears to spring as much or more from the 

 backward displacement of the orbit (especially of its 

 upper border) as from the forward displacement of the 

 molar series. This oblique backward displacement 

 of the orbit may have been correlated with the in- 

 creased size of the nasofacial muscles, and with this 

 factor may also have been correlated the hypertrophy 



