234 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



of premolars moderately developed. These skulls were pre- 

 viously confused by the writer with T. trigonoceras, from which 

 they are readily separated by the horn section, which relates 

 them to some of the primitive types of M. hrachycephalus and 

 equally to M. robustus. The canines are more obtuse than in 



Figure 196. — Type (holotype) skull and lower jaw of Megacerops 



bicornutus 



Am. Mus. 14/6. After Osborn, 1902. One-eighth natural .size. 



M. dispar, and the superior incisors resemble those in Bronto- 

 iherium rather than in M. robustus. 



Etymology. — Named in honor of the late Prof. O. C. 

 Marsh, who estabUshed the remarkable collections of 

 titanotheres in the Yale and National Museums, pro- 

 posed the family name Brontotheridae, gave names to 

 many of the genera and species, and projected the 

 present monograph. 



Present determination. — The species is probably 

 valid. 



Brontotherium leidyi Osborn, 1902 



Cf. Bronlhotherium leidyi, this monograph, page 558 



Original reference. — Am. Mus. Nat. Hist. Bull., vol. 

 16, pp. 105-106, figs. 9, 10, 1902 (Osborn, 1902.208). 



Type locality and geologic horizon. — Big Badlands, 

 S. Dak.; Chadron formation, lower levels of lower 

 Titanotherium zone (Chadron A). 



Type.— A complete skull (Nat. Mus. 4249, skull R) 

 collected by J. B. Hatcher in 1887. (See figs. 198, 

 199.) 



Specific characters. — Osborn writes: 



Nasals elongate, narrowing anteriorly. Horns very short, 

 slightly recurved, of transverse oval section. Canines stout 

 and blunt. Premolars noncingulate, with rounded contours 

 and weU-developed tetartocones. Incisors?^. 



Etymology. — Named in honor of Joseph Leidy, the 

 first of the three great founders of American vertebrate 

 paleontology, describer of Titanotherium, Megacerops, 

 Palaeosyops, author of "The ancient fauna of Ne- 

 braska" and of "The extinct mammalian fauna of 

 Dakota and Nebraska." 



Present determination. — The species is probably 

 valid. 



SPECIES DESCRIBED BY LULL IN 1905 

 Megacerops tyleri Lull, 1905 



Cf. Diploclonus tyleri, this monograph, page 502. 

 Original reference. — Jour. Geology, vol. 13, No. 5, 

 pp. 443-456, pis. 3, 4, August, 1905 (Lull, 1905.1). 



Type locality and geologic horizon. — North side 

 of Spring Draw Basin, about 10 miles from the 

 mouth of Bear Creek, a tributary of Cheyenne 

 River, S. Dak. Type specimen found 35 feet 

 above the base of 200 feet of the Chadron 

 formation {Titanotherium zone) lying upon Pierre 

 deposits, "hence in the upper part of the lower 

 division," as defined by Hatcher in 1893 (1893.1, 

 p. 218). 



Type. — Skull, limbs, and many vertebrae of a 

 single individual (Amherst Mus. 327). (See 

 figs. 200 and 201.) Found by T. C. Brown, of 

 the Amherst College paleontologic expedition of 

 1903. 



Specific characters. — Lull writes: 



Horns well in front of orbits, directed somewhat 

 forward and outward, an elongate oval in basal section 

 with the long axes in line, rounded oval at the summit. 

 Hornlets quite conspicuous, on the inner face of the 

 horns midway between the base and summit. Con- 

 necting crest low and inconspicuous. Nasals broad, 

 well rounded in front, and but sHghtly arched beneath. 

 Zygomata expanded and deep, with a well-rounded outer 

 face. Dentition: Superior incisors represented by the deep 

 and well-defined median alveoli and by the lateral teeth, 

 which remain in place and which have hemispherical crowns 

 which show little sign of wear. The canines are lanceolate, 

 with a well-developed postero-internal cingulum. There is a 

 short diastema in front of, and a longer one behind, the canine. 

 Premolars with a smooth internal cingulum, less pronounced 

 in the middle of the tooth, and with no external cingulum. The 

 deuterocone is well developed, while the tetartocone, especially 

 of premolar 4, is inconspicuous. 



The jaw is deep and robust, with the alveoli of two incisors, 

 probably of the second and third, deep and distinct. There is 



Figure 197. — Type skull of Megacerops marshi 



After Osborn, 1902. Am. Mus. 501. One-tenth natural size. The lower 

 jaw (Am. Mus. 516) figured with this skull does not belong with it. 

 It is probably referable to Brontotherium leidyi. 



'no space between the lateral incisors and the canine, though 

 between the two median alveoli a considerable gap occurs. 

 There seems to have been a small diastema behind the lower 

 canines, which are lanceolate, though with a less prominent 

 cingulum, and not so strongly recurved as the upper ones. 



