VIII CONTENTS 



Plate Page 



XVI. The region of the horn swelling in Palaeosyops, Manteoceras, and Telmatherium 266 



XVII. The region of the horn swelling in Manteoceras, Mesatirhinus, and Dolichorhinus 267 



XVIII. Restorations of the heads of four genera of Oligocene titanotheres 582 



XIX. Incisors and canines of Brontotherium and Teleodus 582 



XX. Upper and lower canines of Oligocene titanotheres 582 



XXI. Left upper premolars of Oligocene titanotheres 582 



XXII. Third left lower molar in Menodus and Brontotherium 582 



XXIII. Juvenile jaw referred by Marsh to Brontops 582 



XXIV. Juvenile jaws and teeth of Oligocene titanotheres 582 



XXV. Superior deciduous and permanent grinding teeth of Menodus giganteus 582 



XXVI. Type skeleton of Eotitanops princeps 702 



XXVII. Mounted skeleton of Palaeosyops leidyi 702 



XXVIII. Restoration of Palaeosyops of the Bridger Basin, Wyo 702 



XXIX. Restoration of Manteoceras and Dolichorhinus of the Uinta Basin, Utah 702 



XXX. Restoration of the skeleton of Dolichorhinus longiceps 702 



XXXI. Manus and pes of Dolichorhinus longiceps 702 



XXXII. Skeleton of Dolichorhinus longiceps 702 



XXXIII. Mounted skeletons of Brontops dispar and Brontops robustus (type) 702 



XXXIV. Mounted skeleton of Brontops robustus (type) , oblique front and side views 702 



XXXV. Mounted skeleton referred to Brontops robustus? 702 



XXXVI. Vertebral column of Brontop srobustus • 702 



XXXVII. Manus and hind limb of Diploclonus tyleri . 702 



XXXVIII. Mounted skeleton of Allops marshi L 702 



XXXIX. Mounted skeleton of Brontotherium hatcheri, left side view ___ 702 



XL. Mounted skeleton of Brontotherium hatcheri, right side view 702 



XLI. Mounted skeleton of Brontotherium hatcheri, front view 702 



XLII. Mounted skeleton of Brontotherium hatcheri, back view 702 



Figure 



1. "Fragment of the inferior maxillary of the left side" of Front's "gigantic Palaeotherium" 1 



2. Type of Palaeotherium? proutii 1 



3. Geologic ages and orogenic periods in North America 2 



4. Successive and overlapping Oligocene and early Eocene formations of the Rocky Mountains 3 



5. Map showing areas throughout the world in which remains of titanotheres have been found and areas in which titano- 



theres were probably in migration during Eocene and Oligocene time 4 



6. The Meek and Hayden Tertiary section of 1862 5 



7. Panoramic section of the Big Badlands of South Dakota, looking southeastward across Cheyenne and White Rivers 



to Porcupine Butte 6 



8. Map showing the type locality of the Titanotherium zone on Bear Creek, S. Dak 7 



9. Map showing cluster of typical lower, middle, and upper Eocene sedimentary basins in the Rocky Mountain region. _ 8 



10. Restorations of Eotitanops borealis and Brontotherium platyceras 10 



1 1 . Ambly poda : Skeletons and restorations of an ancestral and a specialized form 11 



12. Diagram showing the gradual extinction of archaic mammals and their replacement by modernized mammals 14 



13. Phenacodus and Coryphodon drawn to the same scale 15 



14. Contrast between the Linnaean and phylogenetic systems of classification 16 



15. The family tree of the titanotheres 17 



16. Theoretic descent of existing members of the dog family from a common ancestor 19 



17. Successive invasion of nine families of perissodactyls in North America and western Europe 23 



18. Outlines of the body form of the perissodactyls, drawn to the same scale 25 



19. The family tree of the Perissodactyla 26 



20. Periods of expansion and extinction of the perissodactyls and contemporary forms 27 



21. Phyletie divergence in the evolution of new proportions in horses and in titanotheres 29 



22. Contours of the head and of parts of the mouth in browsing and grazing perissodactyls 30 



23. Heads of lower Eocene and modern perissodactyls, showing changes of proportion and of the lip structure 31 



24. Restorations of the heads of some of the principal types of titanotheres 32 



25. Lower jaws of the first and the last of the titanotheres 33 



26. Structure of the feet in extinct and living odd-toed ungulates 34 



27. Restorations of nine species of titanotheres 35 



28. Evolution of the skeleton of the titanotheres 36 



29. Evolution of the skull and molar teeth in the titanotheres 37 



30. Adaptive radiation in the evolution of the upper molar teeth in the perissodactyls 38 



31. Three types of teeth of members of nine typical famihes of perissodactyls 39 



32. The family tree of the perissodactyls, showing adaptive radiation of the nine families and thirty-five subfamilies 40 



33. Outlines of the bodies of titanotheres at different stages of evolution 44 



34. Map showing the known areas and the hypothetical areas of titanothere migration and habitat 45 



35. General geologic sketch map of the Rocky Mountain region, showing existing topography and drainage areas and their 



relation to areas of Eocene and lower Oligocene sedimentation 46 



36. Map of western North America showing supposed routes of migration of animals 49 



37. Map showing the orogeny of the western mountain and plateau region 50 



