In the Swifts, Cormorants, and Penguins, all four toes are directed forwards. In most birds the hallux is directed backwards, and the other toes forwards. In the Owls the fourth toe can be directed backwards as well as the hallux, while in Parrots, Cuckoos, Woodpeckers, and Toucans the fourth toe is permanently reversed. In Trogons the second toe is reversed in addition to the hallux, but not the fourth.
[105] W.K. Parker, _Phil. Trans._ vol. 179, p. 385, 1888; and _Ibis_, 1888, p. 124.
[106] See T.H. Huxley, "On the Classification of Birds," _P.Z.S._ 1867.
The skeleton of the members of this class, the highest of the vertebrata, has the following characteristics:--
Some part of the integument at some period of life is always provided with hairs; these are epidermal structures arising from short papillae of the Malpighian layer of the epidermis, which at once grow inwards and become imbedded in pits of the dermis. Sometimes scales or spines occur, and epidermal exoskeletal structures in the form of hoofs, nails, claws and horns are also characteristic. As regards the endoskeleton, the vertebral centra have terminal epiphyses except in the Ornithodelphia and some Sirenia. In the skull the cranial region is greatly developed as compared with that in lower vertebrates, and whereas in many reptiles the true cranium is largely concealed by a false roof, in mammals the only relic of this secondary roof is found in the zygomatic arch, and postorbital bar. In the adult all the bones except the mandible, hyoid, and auditory ossicles are firmly united together. The basisphenoid is well ossified, and there is no parasphenoid. The pro-otic ossifies, and unites with the epi-otic and opisthotic before they coalesce with any other bones.
The skull articulates with the vertebral column by means of two convex occipital condyles formed mainly by the exoccipitals, and the mandible articulates with the squamosal without the intervention of the quadrate. The latter is much reduced, and is converted into the tympanic ring, while the hyomandibular of fish is represented by the auditory ossicles[108].
The teeth are always attached to the maxillae, premaxillae and mandibles, never to any of the other bones. They are nearly always implanted in distinct sockets, and are hardly ever ankylosed to the bone. The teeth of mammals are generally markedly heterodont, four forms, incisors, canines, premolars, and molars, being commonly distinguishable. Some mammals are _monophyodont_, having only a single set of teeth, but the great majority are _diphyodont_, having two sets, a deciduous or milk dentition, and a permanent dentition.