Many vertebrate animals possess an exoskeleton, formed by a hardening of one or other layer of the integument. The integument is composed of two layers - an external non-vascular "epidermis," and a deeper vascular "dermis" - and the exoskeleton may be formed by the deposition of horny matter, or of salts of lime, in either or in both of these. The epidermal exoskeleton is always horny, and, when present, is generally in the form of hairs (Mammalia), feathers (Birds), scales (serpents and many lizards), or plates (Chelonians). The horny sheaths of the jaws in Birds and some Reptiles, the outer covering of the horns in some Mammals, the hoofs, claws, and nails of Mammalia, are likewise epidermic. The dermal exoskeleton may be either horny or bony; and good examples of it are to be found in the scales of fishes, the bony scutes of the Crocodiles, and the armour-plates of the Armadillos.