EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 



427 



The fine skull in the Field Museum, No. 12187, 

 illustrates the very peculiar constriction of the nasals 

 in front of the orbits and their distal expansion, also 

 the rudimentary horn bosses and curved nasals in 

 lateral view. Riggs notes that 

 the Uinta Basin representatives 

 of M. earlei are somewhat 

 broader headed and more mas- 

 sive than the Washakie Basin 

 type and that in both the Field 

 Museum skulls the hypocone 

 on m' is wanting; there is also 

 a slight elongation of thepostca- 

 nine diastema. (Riggs, 1912.1, 

 p. 30.) 



Dentition. — A feature distin- 

 guishing this animal from both 

 M. megarJiinus and M.fluviatilis 

 is seen in the double convexities 

 (protocone and tritocone) of the 

 ectolophs of the superior pre- 

 molars, as in certain skulls of M. 

 petersoni. The superior molars 

 exhibit prominent ectolophs and 

 greatly elevated internal cones. 

 The internal cingula of p^, p* are 

 large and progressive, extending 

 completely across the internal 

 base of the protocone. The 

 external cingulum of p* also 

 extends nearly across the base 

 of the tritocone. The deutero- 

 cone of p^ is larger than in M. 

 petersoni. The crowns as a 

 whole are laterally compressed. 

 This hypsodont character is 

 an advance on the condition 

 observed in M. petersoni. 



Jaw. — A jaw from Washakie 

 B (Am. Mus. 13179) provision- 

 ally referred to this species, as 

 shown in Figure 358; exhibits 

 proportions which are distinc- 

 tively mesaticephalic, like those 

 of the skull. It is of very su- 

 perior size, and the ramus is of 

 much greater depth than in the 

 jaw referred to M.fluviatilis (fig. 

 358). The canines are short, 

 rounded, and recurved. The 

 hypoconulid of ma is conic and 

 posteriorly cingulate, with a 

 detached cuspule on the base of 

 the inner side. 



Additional ohservations on MetarJiinus earlei. — 

 An excellent skull in the Carnegie Museum, No. 3098, 

 found 190 feet above the bottom of Uinta B 1, has the 

 dimensions following. 



Millimeters 



Basilar length 410 



Zygomatic width 243 



Cephalic (zygomatic) index 60 



P'-m3 162 



pas. pa-'' '"^ 

 Al 



Figure 361. — Type skull of Metarhinus earlei 



One-fourth natural size. Am. Mus. 13166 (type) ; north side of Haystack Mountain, Washakie Basin, Wyo., Washakie B 1. 



Ai, Side view: A2, palatal view; A3, top view; A4, maxillo-premaxillary symphysis, dorsal view; As, occipital view. 



pi-p< 63 



M'-m3 98 



The infraorbital shelf is almost as small as it is in 

 RhadinorMnus, but the nasals are broad. 



