EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 



283 



Measurements of superior molars of Lambdotherium popoagicum, 

 in millimeters 



Other distinctive characters of the superior teeth 

 are the very obUque ectolophs, the prominent para- 

 styles and mesostyles, the sculptured form of the 

 cusps, especially apparent in the unworn specimens. 

 The protoconule is distinct and usually of subtriangular 

 form; it is connected with the protocone by a low 

 crest. The metaconule proper is rather sessile, indis- 

 tinct, or wanting; there is, in fact, a depressed metaloph 

 or rudimentary posterior crest. The third superior 

 molar (Am. Mus. 4664) exhibits an especially oblique 

 ectoloph, also a prominent hypocone and low but 

 distinct metaloph. 



The jaw.— The type jaw (Am. Mus. 4863, fig. 236) 

 consists of two separate and incomplete rami figured 

 by Cope (Tertiary Vertebrata, PL LVIII, B). The 

 distinctive feature of the type species is the inter- 

 mediate condition of the third premolar, which dis- 

 tinguishes this animal from primitive and more pro- 

 gressive specimens. 



There are five more complete jaws, namely, Am. 

 Mus. 14899 (figs. 231, 236), 14905, 14906 (figs. 231, 

 236), 14909, which together afford a full knowledge 

 of the characters of the jaw except the angular and 

 condylar region ; the elongate and laterally compressed 

 chin (fig. 236), even more extreme than that of the 

 Eocene horses; the wide diastema between the canine 

 and the second premolar; the incisive border extending 

 somewhat to support the slender, recurved, prehensile 

 canines and the row of chisel-shaped, semiprocumbent 

 teeth; the coronoid process high, vertically placed, 

 sharply defined, with flat anterior face. 



The extremes of measurement are shown below. 



Measurements of jaw of Lambdotherium, in millimeters 



The premolars are 63 per cent of the length of the 

 molars. 



Lambdotherium primaevum Loomis 



Plate LIV, C, D; text figure 114 

 [For original description and type reference see p. 178] 



Type locality and geologic horizon. — ^Big Horn Basin, 

 Wyo. ; Lamhdotherium-Eotitanops-Coryphodon zone (Big 

 Horn D). 



Specific characters. — Superior molars with crescentic 

 protoconules; cingula completely surrounding the 

 crowns. Measurements as in L. popoagicum. 



This type is significant as coming from the Big 

 Horn Basin. The type superior first and second 

 molar teeth (fig. 114, p. 178) may be readily dis- 

 tinguished by the greater development of the internal 

 cingulum, which completely surrounds the crown. 

 Another feature is that the protoconules are large 

 and subcrescentic, and the metaconules are lost in the 

 metaloph. These measurements^^ are: 



D 



Figure 235. — Upper and lower grinding teeth of 

 Lambdotherium 



Natural size. A, L. popoagicum, Am. Mus. 14902, Alliali Creek, 

 Wolton; left upper premolar-molar series. B, L. priscum. 

 Am. Mus. 14908, Dry Muddy Creek, 18 miles above mouth; 

 light piemolar-molar series. C, L. progressum, Am. Mus. 

 14917 (type). Alkali Creek, Buck Spring; rightlower premolar- 

 molar series. D, L. progTcssum, Am. JNIus. 14918, lower Alkali 

 Creek; first lower molar of the left side, crown view. All 

 from Wind River Basin. 



Measurements of molar teeth of Lainhdolhcrium primaevum 



Millimeters 



M' and m^, combined, anteroposterior 23. 5 



M', anteroposterior 11 



M>, transverse 13 



M^, anteroposterior : 12 



M^, transverse 15 



M^, transverse, maximum along anterior border 18 



Ml to M3, combined, anteroposterior 41 



Ml, anteroposterior 11 



M2, anteroposterior 12. 5 



M3, anteroposterior _17 



22 The measurements of mi and m* were accidentally transposed in Loomis' 

 original description. 



