528 



TITANOTHERES OF AJSTCIENT WYOMING, DAKOTA, AND NEBRASKA 



Transitional skulls jrom M. proutii to M. trigono- 

 ceras. — A skull (Carnegie Mus. 3068 or 558?) assigned 

 to M. trigonoceras proutii appears to be distinguished 

 subspecifically from the more progressive stage M. 

 trigonoceras by the retarded development of the 

 tetartocones of p*, by the more primitive size and 

 section of the horns, by the hypsodont characters of the 

 external cusps of the molars, and by the correspond- 

 ingly deep fossettes. This apparently occupies an 

 intermediate position between the M. Tieloceras of the 

 lower beds, M. torvus, and the M. trigonoceras of the 

 middle beds. The sex is apparently female. To 

 this may be attributed the fact that the horns are 

 somewhat more primitive, elongate-oval in section. 



The skull is that of a young animal in the sixth stage 

 of growth, the protocone of m' being barely worn. 

 As shown in the comparative measurements, the 

 superior grinding teeth are of strikingly dolichocephalic 

 type, the measurement p'-m^, inclusive, being 320 

 millimeters. 



Figure 438. — Upper teeth of Menodus proutii 



Am. Mus. 9335 (neotype); Chadron B, Lance Creek, Wyo.; exhibits the generic characters of Menodus. 

 Canines conical with sharply defined cingulum, both external and internal cingula of the premolars and 

 external cingulum of the molars sharply defined, molars elongate anteroposteriorly. Premolar pattern the 

 same as in Brorttops hrachycepJialus. The measurements of these teeth indicate reference to Menodus 

 prfmtii. One-third natural size. 



In this skull, as in dolichocephalic skulls generally, 

 the bridge over the infraorbital foramen is broad and 

 flat, and the external auditory meatus is widely open 

 below. A wide space separates the postglenoid and 

 paroccipital processes. The dolichocephaly of the 

 dental series is also shown in the spacing of the teeth, 

 which is so considerable that there is a slight diastema 

 between the first superior premolar and the canine; 

 cingula appear between the grinding teeth. P' is a 

 large, actively functional tooth, elongate antero- 

 posteriorly. The canines have the highly charac- 

 teristic form of Menodus (Titanotherium) , being long 

 and pointed, with anterior and posterior cingula; at the 

 same time, their slender section indicates that they 

 belong to a female. Other distinctively Menodus 

 characters are the crenulate internal faces of the 

 deuterocones and the crenulate and strongly developed 

 external and internal cingula with sharp edges. In 

 the grinding teeth the hypocones are very prominent, 

 and both the internal cones and the external crescents 

 exhibit deep slopes; this elongate or subhypsodont 



character of the grinders, distinctive of all true 

 Menodus teeth, is very striking. 



Menodus trigonoceras (Cope) 



{Symborodon trigonoceras Cope, 1873; "Titanotherium trigono- 

 ceras" Osborn, 1902) 



Plates XX, XXII, CXXVIII-CXXXII, CXXXIV, CXXXV, 

 CLVII; text figures 172, 378, 382, 396, 399, 409, 435, 437, 439, 

 440, 613-616, 630, 639 



[For original description and type references see p. 213. For skeletal characters see 

 p. 683] 



Type locality and geologic Tiorizon. — Titanotherium 

 zone, level Chadron B. 



Specific characters. — Skull larger than in M. Tielo- 

 ceras. Premaxillaries to condyles 670 to 725 milli- 

 meters, tip of nasals to top of occiput 670 to 738. 

 Mesaticephalic, width across zygomata 485 to 

 535 (?) millimeters. Index 74 (?)'. Horns, out- 

 side length 132 to 190 millimeters, basal section 

 triangular, tips pyramidal; horns a little in front of 

 preorbital malar ridge. Nasals squared distally, 

 free length 115 to 140 millimeters, 

 breadth 125 to 135 (?). Buccal 

 swelling of zygoma more decided. 

 Premolar-molar series 333 to 360 

 millimeters, premolars 127 to 136, 

 molars 203 to 225, canines, anterior 

 d', 42 to 47. Premolar tetartocones 

 more progressive, tetartocone of p"* 

 more pronounced. 



This animal is directly successive 

 to Menodus proutii and connects 

 this species by a series of "ascending 

 mutations" with M. giganteus. 

 on the measurements of Menodus 

 trigonoceras. — In the typical skulls the premolar- 

 molar series is 83 millimeters shorter than in the type 

 of M. {" Brontotherium") ingens. Several skulls (Am. 

 Mus. 1066, 1067, 1007; Nat. Mus. 4291) which were 

 formerly referred to M. giganteus have the premolar- 

 molar series over 60 millimeters shorter than in the 

 type of M. giganteus and appear to belong rather with 

 M. trigonoceras. Between the largest skull now 

 referred to M. trigonoceras (Am. Mus. 1066) and the 

 smallest referred to 31. giganteus there is a marked 

 difference in the length of the true molar series (23 

 mm.). Hence in our collections at present, with 

 reference to the longitudinal dimensions of the 

 grinding teeth, M. trigonoceras does not quite overlap 

 M. giganteus but is separated by a small but distinct 

 interval. 



M. trigonoceras may be distinguished from Allops 

 serotinus by the greater relative anteroposterior 

 measurements of the true molars, by the lesser width 

 of p*, and by the great length of the nasals, as follows : 



Observations 



