106 Manus of Perissodactyles: A, Left manus of _Tapirus_; B, Right manus of _Titanotherium_; C, Left manus of _Chalicotherium giganteum_ 508 107 Left manus of A, _Coryphodon hamatus_; B, _Phenacodus primaevus_; C, _Procavia (Dendrohyrax) arboreus_ 510 108 Left anterior and posterior limbs and limb girdles of _Uintatherium mirabile_ 516 109 Left femur of an Ox (_Bos taurus_) and of a Sumatran Rhinoceros (_Rhinoceros sumatrensis_) 518 110 Pes of A, a Tapir (_Tapirus americanus_); B, a Rhinoceros (_Rhinoceros sumatrensis_); C, _Hipparion gracile_; D, a Horse (_Equus caballus_) 524 ERRATA. p. 172, note, _for_ 14 _read_ 15. p. 279, description of figure, _for_ Tropidinotus _read_ Tropidonotus. p. 287, description of figure, _for_ shoulder-girdle of sternum _read_ shoulder-girdle and sternum. p. 393, middle of page, _for_ VIII _read_ VII. p. 427, line 2, _for_ Grampus _read_ Killer. CHAPTER I. INTRODUCTORY ACCOUNT OF THE SKELETON IN GENERAL. BY the term =skeleton= is meant the hard structures whose function is to support or to protect the softer tissues of the animal body. The skeleton is divisible into A. The EXOSKELETON, which is external; B. The ENDOSKELETON, which is as a rule internal; though in some cases, e.g. the antlers of deer, endoskeletal structures become, as development proceeds, external. In Invertebrates the hard, supporting structures of the body are mainly =exoskeletal=, in Vertebrates they are mainly =endoskeletal=; but the endoskeleton includes, especially in the skull, a number of elements, the =dermal= or =membrane= bones, which are shown by development to have been originally of external origin. These membrane bones are so intimately related to the true endoskeleton that they will be described with it. The simplest and lowest types of both vertebrate and invertebrate animals have unsegmented skeletons; with the need for flexibility however segmentation arose both in the case of the invertebrate exoskeleton and the vertebrate endoskeleton. The exoskeleton in vertebrates is phylogenetically older than the endoskeleton, as is indicated by both palaeontology and embryology. Palaeontological evidence is afforded by the fact that all the lower groups of vertebrates--Fish, Amphibia, and Reptiles--had in former geological periods a greater proportion of species protected by well-developed dermal armour than is the case at present. Embryological evidence tends the same way, inasmuch as dermal ossifications appear much earlier in the developing animal than do the ossifications in the endoskeleton.