[139] W. Ellenberger and H. Baum, _Anatomie des Hundes_, Berlin, 1891. [140] T.H. Huxley, "Dental and cranial characters of the Canidae," _P.Z.S._ 1880. [141] See p. 392. [142] The dura mater is a membrane which lines the cranial cavity and is formed of tough connective tissue. [143] These are not strictly homologous with the basi-hyal and cerato-hyal of the Dogfish. [144] See note to p. 25. CHAPTER XXII. GENERAL ACCOUNT OF THE SKELETON IN MAMMALIA. THE EXOSKELETON AND VERTEBRAL COLUMN. EPIDERMAL EXOSKELETON. =Hair=, which forms the characteristic Mammalian exoskeleton, varies much in different animals, and in different parts of the same animal. A large proportion of mammals have the surface fairly uniformly covered with hair of one kind only. In some forms however there are two kinds of hair, a longer and stiffer kind alone appearing on the surface, and a shorter and softer kind forming the under fur. In most mammals hairs of a special character occur in certain regions, such as above the eyes, on the margins of the eyelids, and on the lips and cheeks, here forming the vibrissae or whiskers. Sometimes as in _Hippopotamus_, _Orycteropus_ and the Sirenia, the hair, though scattered over the whole surface, is extremely scanty, while in the Cetacea it is limited to a few bristles in the neighbourhood of the mouth, or may even be absent altogether in the adult. In most mammals the hairs are shed and renewed at intervals, sometimes twice a year, before and after the winter. The vibrissae or large hairs which occur in many animals upon the upper lip, and the mane and tail of Equidae are probably persistent. In the hedgehogs, porcupines and _Echidna_ certain of the hairs are modified and greatly enlarged, forming stiff spines. Similar spines occur in the young of _Centetes_, and in _Acanthomys_ among the Muridae. Several other forms of epidermal exoskeleton are met with in mammals, including:-- (_a_) =Scales=. These overlie the bony scutes of armadillos and occur covering the tail in several groups of mammals, such as beavers and rats. In the Manidae the body is covered by flat scales which overlap. (_b_) The =horns= of Bovine Ruminants. These, which must on no account be confused with antlers, are hollow cases of hardened epidermis fitting on to bony outgrowths of the frontals. In almost every case they are unbranched structures growing continuously throughout life, and are very rarely shed entire. In the Prongbuck _Antilocapra_ however they are bifurcated and are periodically shed. Horns are nearly always limited to a single pair, but the four-horned antelope _Tetraceros_ has two pairs, the anterior pair being the smaller.