CONTENTS 



Page 



Chapter VIII. The muscular anatomy and the restoration of the titanotheres, by W. K. Gregorj' 703 



Section 1. Muscles of the face and jaw 703 



Section 2. Restorations of the heads of titanotheres 706 



Section 3. Muscles of the neck and back 709 



Section 4. Muscles of the limbs of the Eocene titanothere Palaeosyops (subgraviportal type) 714 



Section 5. Muscles of the limbs and vertebrae of Oligocene titanotheres 725 



Section 6. Restoration of the musculature and body form of Broniops robusius, an Oligocene titanothere 725 



Chapter IX. Mechanics of locomotion in the evolution of limb structure as bearing on the form and habits of the titano- 

 theres and the related odd-toed ungulates 727 



Section 1. Adaptation to locomotion in the limbs of the fleet (cursorial) and the ponderous (graviportal) types of 



titanotheres and other hoofed quadrupeds 727 



Mechanical and physiological principles governing the proportions and angulation of limb segments in graviportal 



ungulates 727 



Researches made and principles established 727 



Principles of leverage and muscular action, by W. K. Gregory 727 



Straightening of the limbs and arches in adaptation to great weight (graviportal type) 731 



Indices and ratios of limb segments in cursorial and graviportal ungulates (aUometric adaptations) 733 



Results of comparative studies 733 



Ratios showing evolution from primitive to graviportal and cursorial types 735 



Primitive ungulate types 735 



Phyletic progression (Eocene) to graviportal type 736 



Primitive cursorial condylarths and perissodactyls 736 



Mediportal or intermediate group 736 



Phyletic progression to cursorial type in the Equidae 737 



Cursorial light-bodied artiodactyls 737 



Subcursorial deep-bodied artiodactyls 738 



Cursorial heavy-bodied artiodactyls 738 



Graviportal short-limbed digitigrades 738 



Graviportal long-limbed digitigrades 738 



Graviportal rectigrades 739 



Summary of cursorial and graviportal proportions of segments of limbs of ungulates 739 



Features considered 739 



Forms of scapula 740 



Forms of humerus 741 



Analogous adaptation in humerus and femur 742 



Forms of ilium 743 



Length of ilium and of ischium and ratio of these lengths to total length of os innominatum 746 



Ratios of length of scapula and ihum to that of humerus and femur, respectively 746 



Section 2. Systematic comparison of the pectoral and pelvic arches and of the limb bones in eight famihes of peris- 

 sodactyls 747 



Tapiridae and Lophiodontidae 747 



Palaeotheriidae 747 



Rhinocerotidae, Hy racodontidae, Amynodontidae 750 



Equidae '^53 



Brontotheriidae — the titanotheres 754 



Chapter X. Theories as to the origin, ancestry, and adaptive radiation of the titanotheres and other odd-toed ungulates.. 757 



Section 1. Origin and relationships of the Perissodactyla 757 



Nature and habitat of the ancestral perissodactyl 757 



Hypothetical origin of the Perissodactyla from the Condylarthra 758 



Eocene Condylarthra nonperissodactyl 759 



Principal characters of the ancestral perissodactyls 760 



Tetradactyl manus of primitive Perissodactyla 762 



The skull of primitive Perissodact3da 764 



Section 2. Origin and phyletic radiations of the titanotheres and other Perissodactyla 768 



Skull of the primitive titanothere 768 



Structure of the foot in the titanotheres and in other perissodactyls 769 



Mechanics of the perissodactyl manus 774 



Summary of the evolution of the perissodactyl families 776 



Convergence in habitat and habit 778 



Descent of the bunoselenodont families 779 



Descent of the lophodont families 779 



Survival and extinction of the Perissodactyla 780 



Phyletic branching of the titanotheres ■ 780 



