EVOLUTION OP THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 



557 



Geologic succession of lower Oligocene species of Brontotherium 



C. or LARGE SIZE 



Upper Tilanotherium zone: 



Sixth stage, B. platyceras: Grinding teeth 337-340 milli- 

 meters; nasals vestigial; horns extremely broad and flat. 



Fifth stage, B. ramosum: Grinding teeth 350 millimeters; 

 nasals vestigial; horns planoconvex at base, expanding 

 at summits. 



Fourthstage, B. curium: Grindingteeth 346-350 millimeters: 

 nasals shorter; horns slightly convex to plane posteriorly. 



Third stage, B. medium: Grinding teeth 365 millimeters; 

 nasals shorter; horns with prolonged malar ridge; con- 

 cave antero-external faces. 



Second stage, B. dolichoceras: Nasals shorter; horns with 

 incipient malar ridge. 



First stage, B. gigas: Animals of maximum size; grinders 

 330-353 millimeters with cingula; horns longer, oval, re- 

 curved, 275-360 millimeters in height; nasals narrow, 

 more reduced. 



B. OP INTERMEDIATE SIZE 



Middle Tilanotherium zone, upper levels: 



B. tichoceras (level unrecorded) : Grinding teeth 330 



millimeters; horns obliquely oval; phyletic position 



somewhat doubtful. 

 B. hatcheri: Grinding teeth 310-320 millimeters with 



cingula more or less developed; nasals longer than in 



B. gigas; horns 250 millimeters in height. 



A. OF SMALL SIZE 



Level doubtful, either lower or middle Tilanotherium zone; 

 B. hypoceras: Grinding teeth as in B. leidyi (305 mm.); 

 nasals intermediate in length; horns longer (140 mm.), 

 more anterior in position. 

 Upper levels of lower Tilanotherium zone: 



B. leidyi: Grinding teeth 290-300 millimeters, typically 

 smooth and rounded, cingulum faint or wanting; nasals 

 elongate: horns rudimentary, placed above orbits. 



The Brontotherium phylum as represented in the Hatcher collection of 44 skulls and jaws from the Ghadron 

 formation in the United States National Museum 



B. platyceras (Scott and Osborn) . 



Do 



B. ramosum (Osborn) 



B. medium (Osborn) 



B.? medium (Osborn) 



B. curtum Marsh 



Do 



Do 



Do-... 



Do 



Do 



B.? curtum Marsh 



B. gigas? Marsh 



B. gigas Marsh 



B.? gigas Marsh 



Do 



B. gigas Marsh 



B.? gigas Marsh 



Do 



Do 



Do 



Do 



B. gigas? medium? 



B.? gigas Marsh 



B. gigas (hatcheri) Marsh 



B. sp . 



B. medium (?tichoceras) 



B.? tichoceras (Scott and Osborn) 



B. sp "-. 



?B. sp 



B. sp 



B. hatcheri Osborn 



B. medium? (hatcheri) 



?B. hatcheri Osborn 



B. hatcheri Osborn 



B. (hatcheri) hypoceras (Cope)-.. 



B. (hatcheri) hypoceras 



B. hypoceras (Cope) 



Do 



Do 



Do 



B. leidyi Osborn 



Do 



B. (leidyi?) (hypoceras?) 



Catalog No. 



8730 



8729, d' 



1243, 9? 



4256, c? 

 8772 

 8726 



4946, cf 



1232, c? 



1211, c? 



8727, cf 



8728, 9? 

 8743, cf 

 4244, 9? 

 8752 

 8755 

 8758, 9 



8762, 9 

 4903 

 8773, cf? 

 8774 

 8783 

 8787, cf ? 

 8791, c? 

 8800, 9 

 4262, c? 

 8805 

 4699, cT? 

 8313, c? 

 1227, 9 

 8734 

 8775, cf 

 1216, cf 

 4716, 9? 



4255 



4704, 9 



8780, cf 



8789, &1 



4273, c? 



4702, 9? 



8763, 9? 

 8757 

 4249, 9? 

 4250 

 8795 



Material 



Skull. 



Nasals and horns. 



Horns. 



SkuU. Type. 



Pair of jaws. 



Fine skuU and horns. 



Very fine skull. Right horn broken and healed during life. 



Left upper dental series. 



Skull. 



Anterior half of skull. 



Skull and jaws. 



SkuU, nasals, horns (partly destroyed). 



Skull. Horns imperfect. Agrees with B. gigas in dental measurement. 



Upper dentition. Premolars and molars smaller than in large B. gigas. 



Upper dentition. 



SkuU. 



Skull and parts of lower jaws. Horns small and feeble. 



Parts of both rami. 



Pair of lower jaws lacking posterior part of left. Aged individual. 



Pair of lower jaws (young). 



Left ramus. 



Pair of lower jaws. 



Pair of lower jaws. 



Left ramus. 



SkuU, jaws, and part of skeleton. 



Lower jaws and anterior dorsal (young). 



SkuU. 



Skull and jaw (good). 



Lower jaw (immature). 



Indeterminate skull lacking nasals and horns. Poor. 



Pair of jaws (small). 



SkuU. Type. 



SkuU. No canine, horns, or nasals. Genetic reference doubtful; B. hatcheri 



in measurement. 

 Skull with front part of jaw. 



Skull. Small female; horns with connecting crest. 

 Pair of lower jaws. 

 Right ramus. 



Skull. Neotype. Important male skull. 

 SkuU. 

 SkuU. 



Pair of jaws; horn cores; nasals. 

 SkuU. Type. 

 Part of jaw. 

 Pair of jaws. 



