EVOLUTION OF THE SKULL AND TEETH OP EOCENE TITANOTHERES 



293 



Premolars of neotype. — P' is placed midway between 

 the canine and p^ consistently with the relatively 

 elongate preorbital region. The chief features of p^^* 

 are the simple, backwardly directed deuterocones 

 with low crests connecting them with the protocones 

 and tritocones; the tritocones (see figs. 229, 250) in- 

 crease progressively in p'"^; external cingula faintly 

 indicated in p^ p*. The three premolars taken to- 

 gether are subordinate in measurement (36 mm.) to 

 the molars (54 mm.). 



Superior molars oj neotype. — The superior molars 

 exhibit the characteristic 

 bicrescentic ectoloph with 

 prominent parastyles and 

 mesostyles, and median ridges 

 opposite the paracones and 

 metacones (PL LIV, fig. 229); 

 the protoconules are fairly 

 prominent and faintly cres- 

 centic in m'~', forming a ves- 

 tigial protoloph; the internal 

 cingula festoon but do not sur- 

 round the inner sides of the 

 crown; m^ entirely lacks the 

 hypocone; protolophs distinct 

 but sessile are observed on 

 m'"^, also faint rudiments of 

 metalophs on m\ m^ 



Lower molars of referred speci- 

 mens. — The valuable series of 

 jaws (Am. Mus. 14887, 14890, 

 14891, 14888, and 4886) com- 

 plete our knowledge of the 

 inferior dentition except the 

 incisors, which are unknown 

 (figs. 248-250). The premolar- 

 molar series, p2-m3, exhibit 

 progressive gradations of 

 length from 94 to 98 milli- 

 meters. (See table on p. 290.) 

 They are thus superior to E. 

 irownianus and inferior to E. 

 princeps in measurement. The 

 premolars afford the distinctive 

 specific characters or muta- 

 tions in the progressive stages 

 of the internal cuspules or rectigradations. 



Besides the somewhat arbitrary association of the 

 type and neotype, we also refer to this species the 

 materials listed above, including a number of jaws and 

 portions of the skeleton. (See figs. 231, 246, 248, 249, 

 494, 501-503, 505, 521, 522, 701.) 



Characters of the teeth. — The fourth superior pre- 

 molar (Am. Mus. 4892; PI. LIV, H) measures antero- 

 posteriorly 12 millimeters, transversely 14; it exhibits 

 a faint external, distinct anterior and posterior, but no 



internal cingula, conical deuterocone, small proto- 

 conule and larger convex protocone, a somewhat 

 smaller and more plane tritocone and small meta- 

 conule ridge. The superior molars exhibit faint 

 external, more prominent anterior and posterior, and 

 incomplete internal cingula; the ectoloph consists 

 of sharply defined parastyle, paracones and meta- 

 cones with median external ridges, and a prominent 

 mesostyle; the most distinctive feature of the inner 

 half of the crown in m' is the sublophoid character of 

 the protolophs and hypolophs, which unite respec- 



FiGUEE 248. — Lower jaw of Eoiitanops borealis 



One-half natural size. Am. Mus. 14891; west bank of Wind River, 3 miles above canyon (top of banded beds); Wind 

 River formation. Ai, Outer side view; A2, inferior view. 



tively with the distinct protoconule and a much less 

 distinct metaconule to form a low or sessile crest. 

 This rudimentary or vestigial lophoid character is 

 even less evident in the middle Eocene species of 

 titanotheres. M' measures 18 by 17 millimeters 

 (ap. by tr.); it is a nearly quadrate tooth, in wide 

 contrast to the transversely expanded tooth of L. 

 popoagicum. In m^ the less worn paracone measures 

 7 millimeters in height ; the ectoloph is thus somewhat 

 elevated in these molars, but its crescents are not 



