EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 



429 



Metarhinus cristatus Riggs 



[Compare M. fluviaiilis Osborn] 



Plates LXXVIII, LXXIX; text figure 139 

 [For original description and type references see p. 191] 



Type locality and geologic horison. — Uinta Basin, 

 Utah; Metarhinus zone (Uinta B 1), lower levels. 



Specific characters. — Skull, estimated length 380 

 millimeters, breadth 240; molar series 94; frontal 

 region broad; sagittal crest long and high; zygomatic 

 arches relatively heavy. Molars short crowned, no 

 hypocone on m^. 



Materials. — Known only from the type (Field Mus. 

 12194), a single skull lacking nasals and premaxillaries. 



The type (fig. 139) was described by Riggs under 

 the impression that it came from a much higher 

 geologic level than that at which the type of M. 

 fiuviatilis was found. This geologic record has now 

 been corrected to show that M. cristatus was found at 

 nearly the same level as the type of M. fiuviatilis — 

 the lower half of Uinta B 1, or the Metarhinus zone. 

 M. cristatus may therefore be the same animal as 

 M. fiuviatilis. It is true that there is no hypocone 

 on m^ in M. cristatus, but this element is described as 

 a mere "cingulate hypocone" in M. fiuviatilis. The 

 disparity in size between the two types is not very 

 great, the breadth across the zygomatic arches being 

 205 millimeters in M. fiuviatilis and 240 in M. cristatus. 

 The character of the sagittal crest and of the orbital 

 region in the two types is quite similar, although it 

 would appear that in the M. fiuviatilis type the orbits 

 are somewhat more prominent. The difference may 

 be sexual, but the size of the skull of M. cristatus is 

 intermediate between that of M. fiuviatilis and M. 

 eirlei (see measurements above). Its breadth, or 

 cephalic index, is possibly greater, although its length 

 is merely estimated. It certainly contrasts sharply 

 with the long-headed M. riparius. 



Metarhinus riparius Riggs 

 Plates LXXVIII, LXXIX; text figure 138 

 [For original description and type references see p. 191] 



Type locality and geologic horizon. — Uinta Basin, 

 Utah; "Metarhinus sandstones" at summit of Meta- 

 rhinus zone (Uinta B 1). Abundant in both lower and 

 upper levels of Uinta B 1. 



Specific characters (Riggs). — Skull long and narrow, 

 length 405 millimeters, breadth 210, cephahc index 

 55, 51; anterior cranial region expanded, sagittal 

 crest short; interorbital region relatively narrow and 

 rounded; rudimentary horn cores above orbits; man- 

 dible straight in the ramus ; canines large, lower canine 

 long and recurved ; molar series relatively short, 88-93 

 millimeters, hypocone usually present on m^. 



Geologic distribution and materials. — The type of this 

 species (Field Mus. 12186) was found in the "Meta- 

 rhinus sandstones " at the summit of Uinta B 1 , but the 



animal also occurs abundantly in the lower levels, 

 associated with the broad-headed M. cristatus and 

 with Dolichorhinus longiceps (see table, above). The 

 author describes it as the most common species in 

 this life zone. Four skulls, two associated lower jaws, 

 and one isolated jaw in the Field Museum collections 

 are referred to it — namely. Field Mus. 12174, 12183, 

 12191, 12195, 12196. 



General characters. — The type of these species is a 

 laterally crushed skull, a condition which greatly 

 increases its apparent dolichocephaly; this character, 

 however, seems to rest substantially on other speci- 

 mens, the cephalic indices ranging from 51 to 53. It 

 is described as the long-headed Metarhinus riparius 

 and may possibly be ancestral to the aberrant animal 

 described above, known as Sphenocoelus, which it 

 resembles in the following characters: (a) Relative 

 dolichocephaly; (&) form of the occipital and especially 

 of the temporal crests; (c) wide separation of the post- 

 glenoid and post-tympanic processes. It is possible 

 that this represents a branch, M. riparius-Sphenocoelus 

 phylum, which may also be represented in the Prince- 

 ton occiput (Princeton Mus. 10041) from Washakie ?A. 



The hypocone on m^ is not invariably present 

 although seen in the type and in the examples of the 

 species from higher levels. The canines (Riggs, 

 1912.1, p. 29) are said to be strong, whereas in the 

 broad-headed species, M. cristatus and M. earlei, they 

 are reduced in size. 



Metarhinus? sp. 



Text figure 359 



A lower jaw (Am. Mus. 1859), from Uinta B 1, is 

 larger than that of any known Metarhinus or Rhadi- 

 norhinus but probably represents a relative of those 

 genera. It differs from the various species of Doli- 

 chorhinus in being stouter and in having a shorter 

 tooth row. Comparative measurements are given 

 above. 



Another lower jaw (Am. Mus. 2355), from "Glover 

 Spring," level Washakie B, apparently represents a 

 large Metarhinus of uncertain specific reference. It 

 is probably not referable to Dolichorhinus vallidens. 



SUBFAMILY RHADINORHININAE (=?MEGACEROPINAE) 



Animals of medium size; divergent in structure 

 from members of the Mesatirhinus or Metarhinus 

 phyla. Skull with nasals contracted and pointed 

 distally; cranial profile concave instead of convex; 

 skull cyptocephalic ; facial region — that is, grinding 

 series — somewhat upturned; infraorbital processes 

 not prominent ; frontonasal horns wanting or retarded 

 in development; traces of terminal nasal horns in one 

 species. Molars subhypsodont; premolars slightly 

 progressive. 



This group of small titanotheres is contemporaneous 

 with Metarhinus, Mesatirhinus and Dolichorhinus Ion- 



