558 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



Brontotherium leldyi Osborn, 1902 



Plates XIX-XXII, XLVII, CLXI-CLXV; text figures 27, 29, 

 33, 87, 198, 199, 372, 383, 390, 392, 395, 399, 401-403, 405, 

 457, 458, 461, 462, 464, 519, 620, 621, 624, 626-629, 635-637, 

 661, 668, 690, 709, 720, 727 



[For original description and type references see p, 234. For skeletal characters 

 see pp. 691, 697] 



Geologic Tiorizon. — Lower Titanotherium zone of 

 South Dakota, middle level (Chadron B). 



Specific characters. — P'-m' 290-300 millimeters, 

 m'-m^ 114-120, dental index 45. Basilar length of 



therium zone, middle level (B 1), and by a beautifully 

 preserved skull and lower jaws (No. 93) in the Car- 

 negie Museum at Pittsburgh, associated with a 

 humerus, radius, ulna, tibia, and atlas. 



General description. — This species is positively re- 

 corded from the lower Titanotherium zone and is of 

 great interest because Brontotherium leidyi, while very 

 primitive and approaching the primitive members of 

 the other phyla in certain respects, is yet readily dis- 

 tinguished from its contemporaries — first, from Bron- 



WMM0 



D 



A B 



Figure 461. — Sections and contours of skulls of Brontotherium leidyi and B. hypoceras 



Brontotherium leidyi, Nat. Mus. 4249 (type); horns small and placed not far in front of the orbits, their basal section ovoid with the narrow end external, the 

 section of the upper part of the horns roundly oval, connecting crest slight, nasals long and lender, zygomata slightly expanded. B, B. leidyi, Carnegie Mus. 93, 

 a referred specimen with horns longer, basal section obliquely oval, zygomata little expanded, nasals long. C, B. hypoceras, Nat. Mus. 4273 (neotype), horns 

 much longer than in B. leidyi and placed far in front of the orbits, their basal section oval, nasals short. Thus B. hypoceras is a much more advanced stage 

 than B. leidyi. D, B. hypoceras, Nat. Mus. 4702. One-seventh natural size. 



skull 665 mUlimeters, zygomatic breadth 440, index 66 

 (dolichocephalic or mesaticephalic). Nasals elongate 

 (free length 118 mm.), broad proximally (105 mm.), 

 tapering distaUy. Horns low (height above narial 

 sinus 107 mm.), placed above preorbital foramen, 

 recurved, with gently sloping anterior contour, sum- 

 naital section a transverse oval. Zygomata with 

 buccal swelling slight. 



Materials. — The species is represented by the type 

 skull (Nat. Mus. 4249, skull R) from the lower Titano- 



tops hrachycephalus by the greater length and size of 

 the skull as a whole; second, from the type of Allops 

 walcotti (Nat. Mus. 4260) by the more pronounced 

 development of the tetartocones of the premolars, 

 especially of p4, by the reduction or absence of internal 

 and external cingula on the premolars and molars, by 

 the rounded, obtuse contours of the premolars, and 

 especially by the transverse oval section of the horns 

 from base to summit. In all these characters it antici- 

 pates the well-marked B. gigas and B. hypoceras types. 



