EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 



423 



depressed infraorbital shelves. The peculiar elevation 

 of the front of the upper jaw may be partly increased 

 by crushing but appears to be correlated with a similar 

 upward flexure of the front part of the lower jaw (figs. 

 356, 358). The third molar is somewhat farther 

 back with reference to the orbit 

 than in Doliehorliinus. 



Dentition of type and referred speci- 

 mens. — Some of the "referred" 

 specimens are recorded from Uinta 

 B 1 and may well belong to a more 

 progressive species than M. fluvia- 

 tilis. The following description of 

 the dentition includes the referred 

 Uinta B 1 specimens which may 

 belong to more progressive muta- 

 tions or species than the type of M. 

 fluviatilis. 



The teeth of this diminutive 

 titanothere are represented by the 

 very much worn and somewhat 

 crushed series of the type (Am- 

 Mus. 1500, Uinta B 1) and by the 

 perfect grinding teeth of Am. Mus. 

 1946, Uinta B 1, with which is 

 associated the lower dentition ; also 

 by the complete dentition of an- 

 other jaw (Am. Mus. 2059, Uinta 

 B 1) and possibly by a more frag- 

 mentary upper jaw (Am. Mus. 1864) 

 from Uinta B 1. 



Incisors : The six upper incisors, 

 as seen from the lower side, are 

 arranged in a semicircle, much as in 

 DolicTiorhinus. They have smooth, 

 gently convex anterior faces and in- 

 crease gradually in size from i' 

 to i', a tooth which is separated 

 from the canine by a narrow dia- 

 stema. The inferior incisors as 

 exhibited in Am. Mus. 2059 are 

 intermediate between the Mesa- 

 tirJiinus megarhinus and Dolicho- 

 rhinus types, pointed by wear, in- 

 creasing in size very gradually from 

 ii to is, and having smoothly con- 

 cave posterior surfaces bounded 

 by a sessile cingulum. 



Canines: The superior canines 

 are also of the M. megarhinus form 

 and although much crushed in the type exhibit evi- 

 dence of smoothly rounded sides swelling toward 

 the base of the crown and accented by very deli- 

 cate anterior and posterior ridges. The coronal 

 measurements (ap. by tr.) are 17 by 16 milli- 

 meters. In Am. Mus. 1946, although probably a male, 

 101959— 29— VOL 1 30 



the canines are still smaller (ap. 18 mm., tr. 16). The 

 inferior canines in jaw Am. Mus. 1946 are finely pre- 

 served except at the tips, measuring 15 by 14 millime- 

 ters ; they exhibit a strong antero-internal cingulum at 

 the base. In the other jaw (Am. Mus. 2059), obviously 



FiGTJEE 356. — Skull of Metarhinus fluviatilis 



One-fourth natural size. Am. Mus. 1500 (type); White River, Uinta Basin, Utah; Uinta B 1. Ai, Side 



view (crushed downward); Aj, palatal view; As, top view; Ai, occipital view. 



a female, the canines are much smaller (height 18 

 mm., ap. 13, tr. 12), with strong antero-internal 

 cingulum, slightly recurved apex, and somewhat ex- 

 panding base. The incisors and canines as a whole 

 seem to foreshadow the short swollen type seen in the 

 Oligocene Brontotheriinae. 



