190 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



Sthenodectes Gregory, 1912 



Cf. Sthenodectes, this monograph, page 353 

 Original reference. — Science, new ser., vol. 35, No. 



901, p. 545, April, 1912 (Gregory, 1912.1). 



Subsequent reference. — Riggs, New or little known 



titanotheres from the lower Uinta formations: Field 



Mus. Nat. Hist. Pub. 159, Geol. ser., vol. 4, No. 2, 



p. ^8, June, 1912 (Riggs, 1912.1). 



Figure 137. — Type (holotype) skull of Mesatirhinus superior 

 Field Mils. 12188. After Riggs, 1912. Side, top, and palatal views. Less than one-fourth natural size 



Type species. — Telmatherium? incisivum Douglass. 

 Generic characters. — Gregory writes: 



This, genus is distinguished from Telmatherium ultimum Os- 

 born by the following assemblage of characters: (1) The in- 

 cisors are far larger and more advanced in evolution, i' being 

 closely appressed to its fellowr in the median line, with anterior 

 face elongate, antero-internal tip blunt, median basin large, 

 posterior wall or cingulum very massive, i^ i^ extremely large 

 with low recurved tips and very heavy posterior cingula. (2) 

 The postcanine diastema is reduced or absent. (3) Superior 

 premolars 2, 3, 4 are much more advanced than in T. ultimum, 



having very heavy internal cingula, pronounced external cin- 

 gula, high slender internal cusps (deuterocones) ; p^ especially 

 is in a relatively advanced stage as compared with T. ultimum. 



(4) The least tranverse diameters of p* and of the anterior lobe 

 of m' are greater, that of m^ much less, than in T. ultimum. 



(5) The basicranial region differs in many details, such as the 

 apparent junction of the postglenoid and post-tympanic proc- 

 esses below the auditory meatus. (6) The occiput is low, 

 with a sharp, long sagittal crest. (7) The forehead is 



relatively wide. (8) The nasals taper dis- 

 tally. 



From Manteoceras (especially M. uinten- 

 sis) the genus under consideration is dis- 

 tinguished by (1) the form and size of the 

 incisors and canines, (2) the much more 

 advanced stage of evolution of the premo- 

 lars, (3) the shorter anteroposterior diam- 

 eter of m^, (4) the reduction of the post- 

 canine diastema, (6) the arched and 

 spreading zygomata, etc. 



From Dolichorhinus and Mesatirhinus it 

 is separated by the shortness and relative 

 breadth of the skull, the great size of the 

 incisors, the relatively heavy zygomata, 

 and many other details. 



Etymology. — adho^, strength, driKT-qs, 

 a biter; in allusion to the great 

 power and development of the in- 

 cisors and canines. 



Present determination. — A valid 

 genus, offshoot of the typical Telma- 

 therium phylum. 



Mesatirhinus superior Riggs, 1912 



Cf. Dolichorhinus superior (Riggs), this 

 monograph, page 405 



Original reference. — Field Mus. 

 Nat. Hist. Pub. 159, Geol. ser., vol. 

 4, No. 2, p. 26, pi. 6, June, 1912 

 (Riggs, 1912.1). 



Type locality and geologic hori- 

 zon. — White River divide, north- 

 eastern Utah; upper " Metarhinus 

 sandstones," summit of Metarhinus 

 zone (Uinta B 1). (See fig. 137.) 



Holotype.— A skull (Field Mus. 

 12188). 



Specific characters. — Riggs writes: 



Skull 485 by 255 millimeters, molar series 182 millimeters, 

 nasals free to a point over last premolar, infra-orbital process 

 present, arches slender anteriorly, nasals infolded at margins, 

 sagittal area expanded, canines small, p^ and p' oblique to axis 

 of series. Molars relatively small, strong hypocone on m^, pos- 

 terior nares opening opposite the anterior margin of last molar. 



Etymology. — superior, in allusion to its large size 

 and high stage of evolution. 



Present determination. — This is a valid stage im- 

 mediately ancestral to the Dolichorhinus stage. 



