EVOLUTION OF T£E SKULL AND TEETH OP EOCENE TITANOTHERES 



371 



greater than in M. manteoceras and with complete 

 internal cingula; superior canine in female (?) obtuse, 

 recurved, with heavy posterior cingulum; p'-m' 200 

 millimeters (estimated), p^ with deuterocone somewhat 

 better developed, p* (ap. by tr.) 23 by 29; m' 35 by 38, 

 m^ 41 by 43, m' 39 by 42. Face relatively short, 

 zygomata moderate, horn swelling inconspicuous. 



The female type skull (Am. Mus. 13165) of this 

 species was found by the American Museum expedition 

 of 1906 at the base of Haystack Mountain, at the 

 summit of the exposures of level Washakie A. Its 

 decidedly progressive characters beyond those of M. 

 manteoceras, from Bridger D and Washakie A, per- 

 fectly accord with its somewhat higher geologic level. 

 These are displayed chiefly in the canine, premolar, 



ceras (Am. Mus. 1545) from Bridger D, where the 

 length is 423 millimeters. Similarly, from sexual 

 causes the horn rudiments and facial concavities are 

 not pronounced. The posterior sagittal crests are 

 characteristically bifid, or deeply grooved superiorly, 

 terminating anteriorly in the parietal pit which is so 

 distinctive of this species. The occiput is distin- 

 guished by the very decided prominence of the 

 occipital pillars. 



Dentition. — It is the teeth which afford the most 

 marked distinctions of this species. The Cannes 

 (ap. 23 mm., tr. 21) are abbreviate, measuring 24 

 millimeters in length, the tips being worn off. P^ is 

 a compressed, conical, bifanged tooth, measuring 

 (ap. by tr.) 15 by 9 millimeters. P^ (19 by 17) 

 exhibits marked external 

 convexities and a weak ex- 

 ternal cingulum. As there 

 is considerable variation in 

 the strength of the cingulum 

 in M. manteoceras it is un- 

 certain whether the weak 

 cingulum is progressive or 

 not. The deuterocone is 

 more advanced in develop- 

 ment than in the average 

 M. manteoceras. P' (ap. 20 

 mm., tr. 25) exhibits the 

 tetartocone fold somewhat 

 more conspicuously than ;'n 

 the most progressive 

 Bridger D specimens. P^ 

 (23 by 29) is progressive in 

 transverse measurement 

 and in the development of a 

 low, barely perceptible te- 

 tartocone swelling. M' (35 

 by 38) exhibits a prominent 

 internal cingulum, which is 

 almost continuous around 

 the lingual side of the pro to- 

 cone. M^ (42 by 43) shows 

 a strong development of the cingulum (progressive), a and molar teeth. The horns are inconspicuous, prob 



Figure 311. — Skulls of Manteoceras manteoceras and M. washakiensis 



Side view. One-fourtli natural size. A, if. washakiensis , Am. Mus. 13165 (type); base of Haystack Mountain, east end, 

 Washakie Basin, Wyo., upper levels of Washakie A. B, M. manteoceras, Am. Mus. 12678; Henrys Fork Hill, Bridger 

 Basin, Wyo., level C 5; supposed female skull; back of skull slightly raised to correct the vertical crushing. 



crenulation of the enamel, and an elongate ectoloph. 

 M^ (39 by 42) is slightly inferior in size to m^, the 

 cingulum is most pronounced, and there is a well- 

 developed hypocone ridge (progressive) but no dis- 

 tinct hypocone. 



Manteoceras .washakiensis Osborn 



Plate LXVII; text figures 121, 306, 311, 717 

 [For original description and type references see p. 182] 



Type locality and geologic horizon. — Base of Haystack 

 Mountain, Washakie Basin, Wyo.; summit of Uinta- 

 therium- Manteoceras- MesatirMnus zone (Washakie A). 



Specific characters. — Skull somewhat larger than in 

 M. manteoceras (basilar length in supposed female 490 

 mm., estimated). Superior molars and premolars 



ably because the specimen represents a female, and 

 they are smooth rather than rugose. The small size 

 of the canines is also a sexual character, but the form of 

 the canines is very different from that of the specimens 

 in Bridger D and Washakie A in their approach to 

 the obtuse form characteristic of the Oligocene 

 Brontops. 



Progressive characters. — (1) Canines short, obtuse, 

 recurved; (2) internal lobes of p^ and p^ broadening, 

 with shelf for development of deuterocone; (3) a 

 tetartocone spur observed in p', as in most progressive 

 Bridger specimens; (4) very distinct internal cingulum 

 on m'~''; (5) true molar series relatively longer as 

 compared with the premolar series than in M. manteo- 

 ceras, in which an average of six skulls gives the 



