The pes is generally pentedactylate, but in some Crocodiles the fifth digit is vestigial (fig. 48, B), and in some Dinosauria (fig. 35) there are only three digits. The North American Dinosaurs present a continuous series ranging from a pentedactylate plantigrade form like _Morosaurus_, to such a form as _Hallopus_ with a highly digitigrade and specialised pes reduced to three functional digits, and a vestigial fifth metatarsal. The second, third and fourth metatarsals in this form are nearly two-thirds as long as the femur, and the calcaneum is drawn out into a heel much as it is in most mammals. In Lacertilia, Orthopoda and many Chelonia, the ankle joint comes to lie between the proximal and distal row of tarsals as in birds. FOOTNOTES: [91] See pp. 214 and 215. [92] These terms are defined on p. 199. [93] E.T. Newton, _Phil. Trans._ vol. CLXXXIV, B, p. 431 (1893). CHAPTER XVII. CLASS. AVES[94]. Birds form a large and extremely homogeneous class of the vertebrata, and are readily distinguished from all other animals by the possession of an epidermal exoskeleton having the form of feathers. Feathers differ from hairs in the fact that they grow from papillae formed of both the horny and the Malpighian layer of the epidermis, which papillae at first project from the surface, and only subsequently become imbedded in pits of the dermis. A dermal exoskeleton does not occur in birds. The endoskeleton is characterised by its lightness, the large bones being generally hollow; but the pneumaticity does not vary in proportion to the power of flight. The cervical part of the vertebral column is very long and flexible, while the post-cervical portion is generally very rigid, owing to the fusion of many of the vertebrae, especially in the lumbar and sacral regions. The vertebrae are generally without epiphyses to their centra. The cervical vertebrae in living forms have saddle-shaped articulating surfaces, and many of them bear ribs. The thoracic ribs in almost all birds have large uncinate processes. The sternum is very large, and the ribs are always attached to its sides, not as in many reptiles to anybackwardly projecting process or processes. The sternum ossifies from two or more centres.