578 



TITANOTHEEES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



the skull is covered with exostoses, which are partly 

 age or even senescent characters and may represent 

 extreme development of the B. curium type of skull. 

 Among these exostoses are the branching internal 

 expansions of the tips of the horns, to which the name 

 ramosum refers. Remarkable exostoses are observed 

 on the outer and inner portions of the buccal zygo- 

 matic processes and on the rugose tips of the 

 occipital pillars. 



The type skull is vertically crushed; this interferes 



with the natural 

 position of the 

 horns, which are 

 pitched strongly 

 forward, whereas in 

 life the horns must 

 have been vertical 

 or even slightly re- 

 curved, as in well- 

 preserved speci- 

 mens of B. gigas and 

 B. platyceras. Zy- 

 gomatic expansion 

 has now reached a 

 still more extreme 

 stage; the total 

 length of the skull 

 along the basal line 

 is closely estimated 

 at 741 millimeters, 

 while the breadth 

 across the zygo- 

 matic arches now 

 attains 775, the 

 breadth thus ac- 

 tually exceeding 

 the length. The 

 appearance of 

 breadth is, how- 

 ever, greatly in- 

 FiGUBB 479. — Sections and contours of creased by crush- 

 skull of Brontotherium ramosum mg. The molar- 



Am. Mus. 1447 (type). One-ninth natural size. The premolar SCricS is 

 marked forward pitch of the horns, the extreme hqw shortened tO 

 flatness of their basal section, and the extreme „-„ .i,- 



width and flatness of the zygomata have all been oOV millimeters, 

 emphasized by crushing, but even before crushing -wJiick is IcSS than 

 the anterior face of the horns was probably convex, . 



thenasalswereextremely short, and the zygomata m ■^' 7Yie(t%U7Yl. 

 were much expanded. The horns are still 



longer than in the preceding types, measuring 399 

 millimeters (15.7 mches) as compared with 355 in 

 the oldest male of B. gigas and 380 in the type of 

 B. curium. 



A clear separation from the B. gigas type is seen in 

 the frontonasal or sagittal section, which resembles 

 very closely that of the aged B. curium, attaining a ver- 

 tical depth of 160 millimeters. The horn section 

 near the base is, however, quite different from that of 



B. curium, being relatively broader, less convex 

 anteriorly, and quite as flattened or even slightly 

 concave posteriorly. The apical section of the horns 

 is far more flattened and broadened, being very slightly 

 convex on both sides, whereas the apical section of 

 the B. curium type is more similar to the basal section. 

 The external ridge, unlike that in B. plaiyceras, 

 extends down two-thirds of the side of the horn but 

 does not reach the malars in front of the orbits. The 

 nasals are of about the same length as in B. curium. 

 The buccal processes are extraordinarily broad and 

 flat; they exhibit the internal projection toward the 

 temporal fossa, also seen in B. gigas. At the back of 

 the skull the post-tympanic forms a broad and close 

 union, practically a synostosis, with the postglenoid, 

 narrowing down the auditory meatus to a small tube. 

 In the median upper portion of the occiput we do not 

 observe the small pair of tuberosities (fig. 378, F) 

 which characterize B. gigas. The tops of the occipital 

 pillars are grooved by the upgrowth of the rugosities. 



Unfortunately the premaxillaries are broken, and 

 the maxillaries lack the first premolars, which are 

 represented by alveoli. A very striking progressive 

 feature is the complete separation of the tetartocones 

 from the deuterocones in p^-p*, so that although 

 well worn down an enamel isthmus still separates the 

 two areas of dentine. The external cingula of these 

 teeth are obsolete; the internal cingula are ill defined. 



B. ramosum therefore represents a very advanced 

 stage of evolution but does not reach the climax 

 attained in this remarkable series of skulls by the 

 succeeding stage known as B. platyceras. 



The type skull is badly crushed, giving a false ap- 

 pearance of extreme width. In dental measurements 

 it is closer to certain specimens of B. liatcJieri than it 

 is to B. plaiyceras. 



Brontotherium platyceras (Scott and Osborn) 



{Menodus platyceras Scott and Osborn, 1887; "Brontotherium 

 platyceras" Osborn, 1902) 



Frontispiece; Plates XVIII, XIX, CLXXXI, CLXXXII, 

 CLXXXVII-CLXXXIX; text figures 10, 18, 24, 27, 33 

 178, 375, 388, 399, 457-459, 480, 481, 620, 640, 643, 648, 

 649, 661, 707, 725, 726 



[For original description and type references, see p. 221] 



Geologic Tiorizon. — Upper Tiianotherium zone of 

 South Dakota. 



Specific characters. — Dental formula in males I-, 

 C-, P^^, M^. Incisors with rounded, smooth, oval 

 crowns, long axis of crowns anteroposterior, canines 

 (? 42 millimeters. Premolars with external cingula, 

 obsolete, internal cingula reduced; tetartocones fairly 

 distinct. Premolar-molar series 337 millimeters. Skull 

 brachycephalic, 730 by 850 millimeters. Nasals vesti- 

 gial, 20 millimeters. Horns extremely flattened trans- 

 versely, slightly convex posteriorly; deep connecting 

 crest, continuous malar ridge. Zygomata with broad, 

 deep buccal processes. 



