EVOLUTION OF THE SKULL AND DENTITION OP OLIGOCENE TITANOTHERES 



535 



Lower jaw. — Dolichocephaly is also influential in 

 the jaw structure, as among the long-skuUed rhinoc- 

 eroses. The rami of the jaw are long and deep, the 

 lower border being carried horizontally forward until 

 it terminates in a well-marked angulation of the chin; 

 from this point it ascends sharply to the incisive 

 border; the angle is broad, with a marked backward 

 extension. This decided angulation of the chin is 

 quite distinct from the convex lower border and 

 shallow, sloping chin in the Brontotherium series. 

 Another distinctive feature is the greater size of the 

 mental foramen (Am. Mus. 1067) and its more pos- 

 terior position beneath the fourth premolar. In the 

 Brontotherium series the foramen is usually smaller 

 and more anteriorly placed. The angle has a smooth 

 border, is rather delicate and narrow but none the 

 less prominent, flaring obliquely backward with an 

 even curve at the postero-inferior border, in contrast 

 with the Brontofherium angle, which is broader, with 

 a rugose border and truncate apex. The coronoid is 

 broad, has a smooth, rounded anterior edge, and is 

 not strongly curved backward. The smooth backward 

 extension of the angle, the straight lower border, and 

 the definition of the chin are characters exactly 

 paralleled in the dolichocephalic rhinoceroses. 



Variation in size. — This is not only distinctive of 

 sex, the females being more slender in all their pro[)or- 

 tions, but of the stages of growth which are recorded 

 in the measurements of the skulls, and also of the 

 stages of vertical succession or evolution which are 

 recorded in the measurements chiefly of the premolar- 

 molar series, as shown in the tables on page 523. 



Final stage of evolution. — In the Field Museum, 

 Chicago, there is a superb skull (No. P 5927), found 

 near the top of the upper TitanotJierium zone at 

 Phinney Springs, S. Dak. With it were discovered 

 the lower jaws, cervical and anterior dorsal vertebrae. 

 (See p. 686.) The skull is in a beautiful state of pres- 

 ervation and has been very accurately reconstructed 

 by Mr. E. S. Riggs. It belongs to a fully adult 

 animal, with well-worn teeth, and strongly accentuates 

 all the characteristic features of the type of M. {"Bron- 

 totherium") ingens. The lateral and anterior views 

 (Pis. CXXXIX, CXL) accordingly give the most per- 

 fect idea of the extreme development of this phylum. 



Noteworthy peculiarities in the dentition are these: 

 (1) PMs present on the left side of the skull but absent 

 on the right. In most other specimens p' is constant; 

 this, however, proves that in Menodus as in Brontops 

 p' is a variable tooth. (2) Pi is also absent in this 

 specimen, the canine being closely crowded against 

 P2. (3) The third inferior molar exhibits a very small 

 hypoconulid which lacks the internal cup or crescent. 



Noteworthy progressive features are the presence 

 of an anterior swelling or incipient hornlet on both 

 the horns, somewhat similar to that observed in the 

 type of Diploclonus hicornutus, and the very anterior 

 position of the bases of the horns, which are almost 

 101959— 29— VOL 1 37 



as far in advance of the line of the orbits as in the 

 type of Brontotherium gigas. This feature does not, 

 however, interfere with the great length of the nasals, 

 which protrude far beyond the vertical line of the pre- 

 maxillaries. Notable, also, are the relatively great 

 length of the horns (outside measurement 290 mm.). 



The detailed measurements of this important skull 

 are presented in the table on page 523. 



Observations on the Menodus giganteus of the Uni- 

 versity of Wyoming. — The largest dentition among 

 known titanotheres is exhibited in a specimen of 

 Menodus giganteus in the 

 University of Wyoming, 

 from the upper Titano- 

 therium zone of Bates 

 Hole, Carbon County, 

 Wyo. The premolar se- 

 ries (p'-m^) measures 465 

 millimeters, as compared 

 with 410 in the type. 

 The premolars are very 

 large (176 mm.) and highly 

 progressive They con- 

 form in all details to the 

 Menodus type. They 

 show very clearly the 

 enamel folds and ridges 

 which are called "crista," 

 "protoloph," and "meta- 



loph." Their tetarto- „ aa^ a ^- j 



^ Figure 445. — Sections and con- 



cones, although large, are tours of skull of Menodus gi- 

 still connected by a bridge ganteus 



with the deuterOCOneS, as Am. Mus.SOG, 9 (of. fig. 446). in females 



in the Menodontinae ^en- °^ '^'^ species the horns are slender but 



11 rpu J" '^ ha.ve a relatively high connecting crest, 



erally. Ihe dimensions the tasal section is obUquely and roundly 



of the premolars and mo- t"hedral, the nasals are long and are de- 



. ,, curved at the tip. One-eighth natural 



lars are as loilows: size. 



Millimeters 



Pi, ap. by tr 25X26 



P2, ap. by tr 40X47 



ps 51X59 



P^ (estimated) 58X73 



Ml (estimated) 81X77 



M2 (estimated) 100X80 



M^, ectoloph, anteroposterior 114 



M-, transverse (across mesostyle) (estimated) 95 



M^, height of ectoloph (paracone) 88 



MS, ap. by tr. (estimated) 100X78 



M^, transverse (across niesost3'le) 90 



Menodus varians (Marsh) 



{Menoys varians Marsh, 1887; " Titanotheriiim ingens Marsh," 

 Osborn, 1902) 



Plate CXLII; te.xt figures 181, 409, 444 

 [For original description and type references see p. 223] 



Geologic horizon. — Geologic level not known, prob- 

 ably upper Titanotherium zone. 



Specific characters. — The skull is relatively shorter 

 or less extremely dolichocephalic than that of M. 

 giganteus. Skull length, premaxillaries to condyles 



