VI CONTENTS 



Page 



Chapter V. Evolution of the skull and teeth of Eocene titanotheres 251 



Section 1. General principles of the study of the characters of the skuU and teeth 251 



Proportion characters and tendencies of evolution distinguished by analysis and synthesis 251 



Distinctions between proportion characters and new rectigradation characters 251 



Steps in transformation of characters 252 



Proportion and flexures of the skull 254 



Summary as to craniometrj' 255 



Changing proportions of the cranium and face 256 



Cyptocephaly , or f aciocranial flexure 256 



Dolichocephaly, brachycephaly, and correlation 257 



Zygomatic cephalic indices in the titanotheres and other perissodactyls 259 



Relative values of indices 259 



Indices of skulls of Eocene and Oligocene titanotheres 259 



Differences in terminology of skull proportions in titanotheres and in man 260 



Contrast in features of brachy cephaUc and dolichocephalic skulls and teeth 261 



List of abbreviations used in illustrations of skulls -262 



Terminology of the upper molar teeth 263 



Section 2. Introduction to the anatomy of the skuU and teeth of the Eocene titanotheres ._- 264 



Tj-pes of skull of Eocene titanotheres 264 



Feeding habits of broad-headed and long-headed titanotheres 264 



Origin and structure of the "horns" in titanotheres 266 



Proportion and rectigradation in the grinding teeth of Eocene titanotheres 267 



Mechanism of the titanothere grinding teeth 269 



Molarization of the premolars 270 



Correlation of dimensions of upper and lower teeth 272 



Geologic succession and geographic distribution of the Eocene titanotheres 272 



Section 3. The lower Eocene titanotheres 273 



Ancestral titanotheres of the Lambdolherium zone of Wyoming at the end of lower Eocene time 273 



Physiographic environment at the end of lower Eocene time 273 



Contrasts and resemblances between Lambdotherium and Eotitanops 276 



Explorations and discoveries 279 



Systematic descriptions of the lower Eocene titanotheres 279 



Section 4. The middle and upper Eocene titanotheres 297 



Phyla distinguished 297 



Species of Palaeosj'opinae and Dolichorhininae from the upper Huerfano {Trogosus zone) 297 



S3'stematic descriptions of the middle and upper Eocene titanotheres 297 



The palaeosy opine group 297 



The Manteoceras-DolichorJiinus group 357 



Successors to the Manteoceras-Dolichorhinus group 434 



Chapter VI. Evolution of the skull and dentition of OHgocene titanotheres 443 



Section 1. Review of the environment, geologic succession, and geographic distribution of the lower Oligocene titano- 

 theres 443 



Section 2. Introduction to the anatomy of the skull and the dentition of the Oligocene titanotheres 444 



Horns : transformation, elongation 444 



Nasals : expansion, abbreviation 446 



Zygomatic arches: expansion, buccal plates 446 



Occipital pillars : auditory meatus 446 



Sexual characters common to all phyla 448 



Teeth : distinctive features and evolution 448 



Development of the skull and dentition 451 



Summary of the replacement of the teeth in OUgocene titanotheres 455 



Stages of wear of the adult grinding teeth 456 



Age and other characters common to both sexes of titanotheres of all stratigraphic levels 456 



Section 3. Division of the Oligocene titanotheres into groups and subfamilies 457 



Characters of the skuU and teeth of the menodontine and brontotheriine groups 457 



Characters and relations of the subfamilies 465 



Possible Eocene ancestors of the brontotheriine group — 468 



Section 4. Oligocene genera accepted as vahd in this monograph 469 



Section 5. The menodontine group 470 



Subfamily Brontopinae, including the phyla Manteoceras, Protilanotherium, Teleodus, Brontops, and Diploclonus... 470 



Stratigraphic level and distinguishing features 470 



Subfamily characters of Teleodus, Brontops, and Diploclonus 471 



Comparisons and contrasts 471 



Conspectus of characters of the subfamily ' 477 



Conspectus of characters of species 478 



Measurements of the Brontops series 479 



Systematic descriptions of genera and species in the Brontops-Diploclonus phylum ; 481 



