EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 



419 



Sphenocoelus uintensis Osborn 



Text figures 111, 354 

 [For original description and type references see p. 175] 



Type locality and geologic horizon. — Wagon Hound 

 Bend, White River, Uinta Basin, Utah; Metarhinus 

 zone (Uinta B 1). 



Specific characters. — Occipital condyles of striking 

 breadth (131 mm.); occiput of moderate 

 height (130 mm.), of considerable breadth 

 (117 mm.); width across zygomatic arches 

 (230 mm.) about the same as in D. 

 hyognathus. The sagittal crest short (89 

 mm.) and thin (8 mm.) in posterior cranial 

 region. Diverging, rounded supratemporal 

 ridges. 



Materials. — The only specimen known 

 is the type skull in the American Museum 

 (No. 1501). The most striking fact about 

 Sphenocoelvs is that although more special- 

 ized in certain points than Dolichorhinus 

 it is geologically older than D. hyognathus, 

 as it comes from Uinta B 1. Sphenocoelus 

 may at present be regarded as a descendant 

 of a certain type of Mesatirhinus (such 

 as Princeton Mus. 10041, a brain case) 

 which retained the narrow occipital crest 

 but otherwise paralleled Dolichorhinus. 



The sphenoid pit measurements are ap- 

 proximately as follows: Length 41 milli- 

 meters; width 13; depth 22. They are 

 distinctly roofed over dorsally with bone. 

 The function of these basicranial pits is 

 entirely conjectural. Nothing similar has 

 been observed in other Perissodactyla. 



The occipital condyles exceed in width 

 (131 mm.) those of even the very broad 

 type of i>. hyognathus. The glenoid facets 

 exhibit a long, obliquely transverse exten- 

 sion, so characteristic oiD. hyognathus; the 

 postgienoid processes also have a peculiar 

 obliquity. In front and to the outer sides 

 of the postglenoids the squamosals are 

 deeply concave. 



Conclusion. — The peculiar pits remove 

 this animal from any other known genus or 

 species; it is possible that they are not con- 

 stant characters. The various distinctive 

 characters, while somewhat extreme, ap- 

 pear to be paralleled or foreshadowed in 

 the member of the Mesatirhinus-Dolichorhinus series. 

 Eometarhinus Osborn 



Text figures 156, 355 

 [For original description and type reference see p. 200] 



This recently discovered Eometarhinus is recog- 

 nized as ancestral to Metarhinus and is thus the ear- 

 liest known member of the Metarhinus phylum. 



Geologic horizon. — The type specimen was found 205 

 feet below the top of the Huerfano formation, in 

 Huerfano B {Eometarhinus-Trogosus zone). 



Generic chai^acters. — Small; ancestral to Metarhinus; 

 with rudimentary frontonasal horn; nasals elongate; 

 overhanging premaxillaries, decurved as in Metarhinus; 

 no infraorbital shelf; characters apparently interme- 



FiGUEE 355. — Type skull of Eometarhinus huerfanensis, from Huerfano B 



One-half natural size. A, nasals, top view; Ai, anterior nasal sections; A2, posterior nasal sections; B, 



0, palatal 1 



side 1 



' witli crown view of dentition. 



1 rudiments. 



diate between those of the Metarhinus and Mesati- 

 rhinus phyla. 



Type species. — Eometarhinus huerfanensis. (See 

 below.) 



Original description. — Osborn writes (1919): 



This new genus and species from the upper Huerfano is 

 founded upon the anterior portion of a skull (Am. IMus. 17412) 



