Among the striped species, or zebras and quaggas of Africa, the large Grevy's zebra (Equus grevyi) of Somaliland and Abyssinia stands apart from the rest by the number and narrowness of its stripes, which have an altogether peculiar arrangement on the hind-quarters, the small size of the callosities on the fore-legs, the mane extending on to the withers and enormous rounded ears, thickly haired internally.
Small round or oval plates of horny epithelium called " chestnuts," callosities growing like the hoof from enlarged papillae of the skin, are found on the inner face of the fore-arm, above the carpal joint in all species of Equidae, and in the horse (E.
They have large cheek-pouches, large naked callosities, often brightly coloured, on the buttocks, and short thick limbs, adapted rather to walking than to climbing.
Callosities, or bare patches covered with hardened and thickened epidermis, are found on the buttocks of many apes, the breast of camels, the inner side of the limbs of Equidae, the grasping under-surface of the tail of prehensile-tailed monkeys, opossums; &c. The greater part of the skin of the onehorned Asiatic rhinoceros is immensely thickened and stiffened by an increase of the tissue of both the skin and epidermis, constituting the well-known jointed " armour-plated " hide of those animals.
It has been suggested that the above-mentioned callosities or " chestnuts " on the limbs of horses are vestigial scent-glands; and it is noteworthy that scrapings or shavings from their surface have a powerful attraction for other horses, and are also used by poachers and burglars to keep dogs silent.