EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHEEES 



531 



zygomata 515-553 millimeters, index 62-70. Horns, 

 outside length 150-290 millimeters; basal section tri- 

 angular, tips pointed; horns much in front of antorbital 

 malar ridge. Buccal swelling of zygoma about as in 

 M. trigonoceras (that is, moderate). Premolar-molar 

 series, 360-428 millimeters; premolars 133-162, molars 

 223-270; dental index 51 (typical). Premolar tetar- 

 tocones very progressive but still connected with 

 deuterocones by a narrow strip, tetartocone of p* very 

 pronounced. Canines c? very long (70 mm.). 



Menodus giganteus was certainly a contemporary of 

 its long-horned rival Brontotherium gigas and probably 

 extended up to the period of the still more specialized 

 Brontotherium platyceras. While far inferior in length 

 of horn to those brontotheres, it was certainly superior 

 to them in the possession of a superb series of grinding 

 teeth with connecting crowns and more elevated crests 

 and cones. 



The sexual characters of M. giganteus are very 

 clearly brought out by comparison of two male skulls, 

 Nos. 1066 and 505, and two fe- 

 male skulls with jaws, Nos. 1067 

 and 506, in the American Mu- 

 seum collection. A fine skull in 

 the National Museum (No. 1220) 

 also belongs to a male. The 

 skull Nat. Mus. 4291 is recorded 

 from the highest level of the 

 upper TitanotJierium zone. The ' 

 male skulls exceed the female in 

 the width of the zygomatic 

 arches, thus proving that, as in 

 the brontotheres, the buccal proc- 

 esses are partly secondary sexual 

 characters. The comparatively 

 slender, pointed horns, and 

 pointed canine teeth of the fe- 

 males are well exhibited in 

 Plate CXXXV, B. This figure 

 shows incidentally the extreme 

 effects of lateral crushing as con- 

 trasted with the extreme effects 

 of vertical crushing and deformation of the skull. 



The type specimen of the species "B. ingens" was 

 erroneously employed by Marsh to complete the 

 characters of the genus Brontotherium, but we now 

 know that the genotype of Brontotherium — namely, 

 B. gigas — is a very different animal in many ways. 

 The type specimen oi"B. ingens " in the Yale Museum 

 is a fine representative of the species, as described in 

 detail below. The absence of the premaxillary bones 

 in this specimen led to many errors regarding the 

 condition of the incisor teeth; we now have conclusive 

 evidence that in this species, as in M. trigonoceras, the 

 incisors are atrophied, vestigial, or entirely wanting. 

 The evidence as to the character of this species afforded 

 by the type specimen is now supplemented by that of 



an exceptionally fine series of skulls in the American, 

 National, and Field Museums. 



Observations on the measurements oj Menodus gigan- 

 teus. — The true molars of the fragmentary type lower 

 jaw agree in measurement and other characters with 

 jaws referred to this species. 



In the referred skulls the grinding teeth range from 

 385 to 465 millimeters in length, a difference of 80 

 millimeters. The largest skull (in the University of 

 Wyoming) has a longer dentition than any other 

 known titanothere (p'-m^ 465 mm.). 



The extreme contrasts between the narrow mesati- 

 cephalic skull and dentition of Menodus giganteus and 

 the broad (brachycephalic) skull of Brontotherium 

 platyceras are shown in the table below. 



Figure 441. — Restoration of Menodus giganteus 



By Charles R. Knight. About one-ninth natural size. 



Measurements of Menodus giganteus, Brontotherium platyceras, 

 and B. gigas, in millimeters 



P'-m3 



Pi-p* 



M'-m3 



Molar index 



P'', ap. by tr 



M', ap. by tr 



Pmx to condyles- 

 Zygomatic index. 



Nasal length 



Horn length 



425 



150 



270 



32 



825 



62 



175 



290 



58X73 

 100X78 



B. platy- 

 ceras, Field 

 Mus. 12161 



340 



120 



223 



25 



80 



38 



■390 



47X72 

 91X99 



' Estimated. 



