EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERBS 



555 



As thus arranged the skulls form a series in which 

 the horn length and the zygomatic width increase, 

 whUe the free portion of the nasals becomes extremely 

 abbreviated. But the length of the whole grinding 

 series in the later species does not increase pari passu 

 with the size of the skull and with the length of the 

 horns. From B. leidyi to B. medium it increases 

 rapidly from 300 to 365 millimeters, but after that, 

 in B. curium and B. platyceras, the measurement 

 p'-m^ falls to or below 350. Meanwhile the true 

 molars also, which had increased from 190 to 241 

 millimeters, fall to 223. If this apparent falling off in 

 the increase of the grinding series as a whole shall be 

 confirmed by much more extensive material it may 

 indicate that the excessive increase in the horns was 

 detrimental to further increase in the size of the grind- 

 ing series (Osborn). The lengthening of the premolar 

 series is arrested by the shortening of the face, but the 

 widening of the premolars as well as of the molars 

 affords a compensatory increase in grinding area. The 

 length of the true molars as compared with the basal 

 length of the skull changed very little, as shown below: 



Molar index in species of Brontoiherium 



B. platyceras: 



Field Mus. 12161 (basilar length extreme) 25 



Am. Mus. 1448 (basilar length shortened by crushing). 30 



B. ramosum, Am. Mus. 1447 (type) 29 



B. curtum: 



Yale Mus. 12013 (type) 29 



Nat. Mus. 4946 26 



B. medium, Nat. Mus. 4256 (type) 28 



B. gigas: 



Am. Mus. 492 29 



Nat. Mus. 4244 29 



B. "hypoceras," Nat. Mus. 4702 29 



B. leidyi: 



Carnegie Mus. 93 28 



Nat. Mus. 4249 28 



The table of measurements above needs extension 

 from additional material and revision with reference 

 to the length of nasals and of horns. 



SYSTEMATIC DESCRIPTIONS OF GENERA AND SPECIES IN 

 THE BRONTOTHERIUM PHYLUM 



Brontotherium Marsh, 1873 



{Titano-ps Marsh, 1887; 



Plates XVIII-XXII, 

 CXXIV, CXXXII, 

 CCXXXV; text figures 

 174, 177, 178, 182, 191, 

 375, 377, 378, 382, 383, 

 481, 515, 519, 620-640, 

 690, 707-710, 719-727, 



[For original description and ty 



'Brontotherium Marsh," Osborn, 1902) 



XXXIX-XLII, XLVII, LXXXII, 

 CLVII, CLXI-CXCIV, CCXXX- 



■ 10, 18, 21, 24, 25, 27, 29, 33, 87, 165, 

 193, 194, 198, 199, 202, 212, 229, 372, 

 388, 390, 392-395, 398-405, 407, 457- 

 643, 648, 649, 652, 661, 662, 668, 688, 

 740, 744, 746 



pe reterences see p. 209. For skeletal characters 



Generic cTiaracters. — Incisors in males persistent, 

 with large posterior cingula, (?) variable in females; 

 canines large, obtuse; grinding teeth with retrogressive 

 cingula; premolars with progressive tetartocones. 

 Skull mesaticephalic to brachy cephalic (zygomata). 

 Horns diverging laterally, typically with connecting 

 crest, shiftuig forward, oval to flattened in transverse 

 section. 



General characters. — Characters 3-9, 14, 15, 16, 

 18, 19, 21, 22, 23, 24, 28, 29, below, are correlated 

 with brachycephaly. (1) 1 14- cf, incisors c? large, 

 flat-crowned, with stout posterior cingulum; 9 ?one 

 or both pair absent. (2) Canines d' short, robust, 

 obtuse, with stout posterior cingulum; 9 slender, 

 small, with posterior face fiat. (3) Grinding series 

 arched (curvilinear). (4) Upward flexure of pre- 

 molar series as seen in side view decided. (5) Length 

 of premolar-molar series 290-365 millimeters, dental 

 index 42-46. (6) Premolar series short, 114-140 

 millimeters. (7) Internal cusps of grinding teeth 

 low, robust, well rounded, ectolophs sharply depressed 

 to the crowns of the teeth. (8) P*, m^~^ very broad, 

 anteroposterior diameter of m^ and m' less than trans- 

 verse diameter. (9) No cingula between grinders, 

 premolar cingula sessile or absent. (10) P|^, p^ 

 perhaps more frequently absent than in Menodus 

 and Brontops, subquadrangular, often with well- 

 developed tetartocone, outer wall not overlapped 

 posteriorly by ectoloph of p^. (11) Premolar tetarto- 

 cones, cf exhibiting early and pronounced develop- 

 ment, 9 moderate development, tetartocones large 

 and rounded, set well in toward the center of the 

 crown. (12) Premolars with internal cingula, c? 

 blunt, reduced or absent, 9 as in Brontops, external 

 cingula variable. (13) Molars without internal 

 cingula, external cingula faint or absent. (14) 

 Hypocone of m^ prominent, sometimes triradiate. 

 (15) Basilar length of skull, 665-830 millimeters; 

 proportions mesaticephalic to hyperbrachycephalic 

 (index 66-110). (16) Facial portion of skull abbrevi- 

 ated, with premaxUlaries reduced. (17) Cranial 

 part of skull elongate. (18) Preorbital malar bridge 

 narrow, with median ridge prominent and well 

 rounded. (19) Infraorbital foramen iaconspicuous 

 in side view. (20) Malar below postorbital process 

 subflat to round. (21) Free nasals tapering, progres- 

 sively abbreviated and finally vestigial. (22) Horns 

 of medium to extreme size, shifting forward progres- 

 sively, finally extreme, basal section progressively 

 becoming a transverse oval, summit of horn becoming 

 broad, flattened anteroposteriorly, oval in section. 

 (23) Zygomata strongly arched, buccal expansion 

 progressive, in section finally excessively broad and 

 flattened (c?). (24) Midparietal convexity pro- 

 nounced. (25) Occiput cf extremely produced back- 

 ward behind zygomata, 9 much less produced. 

 (26) PUlars flaring moderately to extremely, occiput 

 strongly indented, with median Icnobs. (27) Basi- 

 sphenoidal rugosity variable, vomerine septum vari- 

 able. (28) Postglenoid cf very broad. (29) Jaw 

 robust with shallow more or less concave chin, coro- 

 noid rather slender, tapering, anterior border rounded 

 or square in section, strongly recurved or hooked, 

 pointed at extremity, angle depressed and rugose (d^). 



Incisors reduced in females. — There is some evidence 

 for the absence or reduction of the upper and lower 

 incisors in females; this evidence rests chiefly on the 



