422 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



diminutive measurements and probably representing 

 higher mutations or species occur in Uinta B (Uinta B 

 2 of this monograph) according to Peterson's field rec- 

 ords (1893)— namely, Am. Mus. 1946, 1864, 2059 (a 

 tiny jaw), 1865. Riggs, however (1912.1, p. 21), reports 

 no species of Metarhinus in Uinta B (Uinta B 2 of 

 this monograph), nor have we found any specimens 

 referable to M. fiuviatilis in upper levels of Uinta B 1 . 

 It is possible that the line between B 1 and B 2 is not 

 drawn at the same point by these two observers. 



General characters and Tiabits. — The type skull (Am 

 Mus. 1500) is supplemented by a crushed skull (Am. 

 Mus. 1877). Of the two skulls known neither in- 

 cludes the complete nasals nor affords a knowledge 

 of the nasofrontal horn. We can not therefore speak 

 positively as to this character. A further knowledge 

 of these animals is afforded by Am. Mus. 1946, from 

 Uinta B 1 (see geologic note above), which includes 

 a palate with superior teeth associated with a jaw 

 and parts of the pes and of the limbs. Also recorded 

 from Uinta B 1 we find a series of upper teeth, which 

 differ from those of the type in that the external 

 cingulum is absent and the tetartocones on p^ are 

 somewhat more advanced. From Uinta B 2 also are 

 recorded the American Museum jaws 1865 and 2059. 

 The latter is a very small jaw. 



This animal is by far the most diminutive of the 

 known upper Eocene titanotheres — in fact, it may be 

 described as a dwarfed form. The specific name, M. 

 fiuviatilis, was assigned by Osborn on the ground that 

 the animal was probably a river-living animal. 

 In size it is appreciably smaller than the known 

 specimens of M. earlei, although the teeth are larger 

 than those of the doubtfully referred Metarhinus 

 Junius from the Bridger. 



Specific distinctions. — Distinctions from Metarhinus 

 megarhinus are foimd in both the progressive and the 

 adaptive characters. In its progressive characters, 

 although it is an animal of inferior size, M. fiuviatilis 

 presents an advance upon M. megarhinus in the 

 increased hypsodonty of the molar teeth, the increased 

 strength of the external cingulum, the decidedly sharp 

 and compressed parastyle, the upward curvature of 

 the anterior.portion of the grinding series, a character 

 pointing toward the Oligocene brontotheres. The 

 protoconids or anterior crests of p2_4 are ridged, and 

 the cusps corresponding to the metaconid are better 

 developed. The tooth progression is thus parallel 

 with that of Dolichorhinus in certain respects, divergent 

 in others. 



The adaptive characters are most interesting. We 

 observe especially that the narial openings are carried 

 very far back on the sides of the face, so that a very 

 narrow space is left between the orbits and the narial 

 notch (a feature observed also in M. earlei and 

 Rhadinorhinus diploconus). The region across the 

 orbits shows unusual breadth (partly attributable to 



crushing), because the orbits as a whole are prominent 

 and the circumorbital ring appears to be elevated and 

 the eye sockets themselves are small. This feature 

 is analogous to that in the Oligocene brontotheres, 

 in which the orbits are small. We might therefore 

 conjecture that these animals had adopted aquatic 

 habits, because although the orbits are invariably 

 prominent, the eyes tend to become smaller in all 

 swimming ungulates. A pes provisionally referred to 

 M. fiuviatilis (from Uinta B 1) has slender metapodials, 

 which would indicate cursorial rather than amphibious 

 habits. The habitat must be left an open question 

 until the skeleton becomes definitely known. 



Correlated with a relative shortening and broad- 

 ening of the skull is the narrowing of the occipital 

 condyles. It is difficult to discover the reason for 

 the elongation of the maxillary symphysis. The 

 symphysis appears to be elongate partly because of 

 the deep recession of the narial openings. 



Detailed description. — The skull of this species 

 exhibits a great number of interesting characters, 

 because it presents a wide departure from the skull 

 of all other Eocene titanotheres except those of the 

 little group to which it belongs. It is a cm-ious mix- 

 ture of adaptive, progressive, and conservative charac- 

 ters. Among the last may be mentioned the relative 

 elongation and height of the sagittal crest. The skull 

 is sharply characterized specifically by the combination 

 of the above characters with a prominent infraorbital 

 shelf, a prominent antorbital bridge, and a prominent 

 postorbital process. The superior view of this peculiar 

 skull exhibits the elongate symphyseal union of the 

 premaxillaries, recalling that of Dolichorhinus, the 

 broadened posterior portion of the nasals, the deep 

 lateral depression of the face, or antorbital fossae in 

 front of the orbits, the prominent lateral projection 

 of the orbits, the relatively short supratemporal ridges, 

 which rapidly unite posteriorly, the prominent, narrow 

 sagittal crest. The general resemblance of this 

 aspect of the skull to that of Rhadinorhinus diploconus 

 is quite apparent. The palatal view brings out dis- 

 tinctive features. The posterior narial space is long 

 and narrow with parallel sides, the palatines not 

 projecting inward as in Manfeoceras manteoceras. As 

 compared with that of R. diploconus it appeal's to 

 afford the following important differences: In M. 

 fiuviatilis the basicranial region appears to be less 

 dolichocephalic, the zygomata are relatively heavier, 

 the infraorbital shelf (wanting in R. diploconus) is very 

 prominent. The anterior view of the skull exhibits 

 a prominence above the orbit which possibly represents 

 a supraorbital frontonasal horn element; it may, 

 however, be due to crushing. The lateral aspect of 

 the skull is equally distinctive, as it exhibits the deep 

 narial notch separated from the orbit by a very narrow 

 interval, the prominent antorbital bridge concealing 

 the infraorbital foramen, and the sharp and deeply 



