This second family of the section Hystricomorpha exhibits the conversion of the hairs into spines in perfection, the whole upper part of the body being in several instances completely covered with long, hollow, pointed quills, whilst in all cases great numbers of spines and stiff bristles are mixed with the hair. The form of the skull in these animals is distinctive. It is ovate, the cranial portion being more or less inflated by air-cavities in the bones, and the facial portion short, but the occipital or hinder surface is usually nearly perpendicular; the malar portion of the zygoma has no angular process as in the preceding family; the molar teeth are four in number on each side in each jaw; and the limbs are about equal in development. The incisor teeth are large and powerful. With regard to the development of the tail there are considerable differences, some species having that organ quite short, while in others it is of moderate length, or long and sometimes prehensile. [Illustration: SKULL OF THE PORCUPINE.] The Porcupines fall readily into two distinct groups (sub-families) characterised by structure, habits, and geographical distribution. In the strictly terrestrial species, or True Porcupines (HYSTRICINÆ), which inhabit the warmer parts of the eastern hemisphere, the skull is rather more elongated than in the others; the front margin of the orbit is over the third molar; the molars are rootless when young, but become closed after a time, and the clavicles are imperfect. The upper lip is furrowed; the tail, which may be either long or short, is never prehensile; the soles of the feet are smooth; and the female has six teats. [Illustration: COMMON PORCUPINE.] The arboreal species (SPHINGURINÆ), which are all American, have the skull peculiarly short, the front margin of the orbit over the first molar, the molars always rooted, and the clavicles perfect. The upper lip is not furrowed; the tail is moderate or long, and generally prehensile; the soles of the feet are covered with wart-like tubercles; and the female has only four teats.