Skeletal structures may be derived from each of the three germinal layers. Thus =hairs= and =feathers= are =epiblastic= in origin, =bones= are =mesoblastic=, and the =notochord= is =hypoblastic=. The different types of skeletal structures may now be considered and classified more fully. A. EXOSKELETAL STRUCTURES. I. EPIBLASTIC (epidermal). Exoskeletal structures of epiblastic origin may be developed on both the inner and outer surfaces of the Malpighian layer of the epidermis[1]. Those developed on the outer surface include =hairs=, =feathers=, =scales=, =nails=, =beaks= and =tortoiseshell=; and are specially found in vertebrates higher than fishes. Those developed on the inner surface of the Malpighian layer include only the =enamel= of teeth and some kinds of scales. With the exception of feathers, which are partly formed from the horny layer, all these parts are mainly derived from the Malpighian layer of the epidermis. =Hairs= are slender, elongated structures which arise by the proliferation of cells from the Malpighian layer of the epidermis. These cells in the case of each hair form a short papilla, which sinks inwards and becomes imbedded at the bottom of a follicle in the dermis. Each hair is normally composed of an inner cellular pithy portion containing much air, and an outer denser cortical portion of a horny nature. Sometimes, as in Deer, the hair is mainly formed of the pithy portion, and is then easily broken. Sometimes the horny part predominates, as in the bristles of Pigs. A highly vascular dermal papilla projects into the base of the hair. =Feathers=, like hairs, arise from epidermal papillae which become imbedded in pits in the dermis. But the feather germ differs from the hair germ, in the fact that it first grows out like a cone on the surface of the epidermis, and that the horny as well as the Malpighian layer takes part in its formation. =Nails=, =claws=, =hoofs=, and the =horns of Oxen= are also epidermal, as are such structures as the =scales= of reptiles, of birds' feet, and of _Manis_ among mammals, the =rattle= of the rattlesnake, the =nasal horns= of _Rhinoceros_, and the =baleen= of whales. All these structures will be described later. =Nails= arise in the interior of the epidermis by the thickening and cornification of the stratum lucidum. The outer border of the nail soon becomes free, and growth takes place by additions to the inner surface and attached end. When a nail tapers to a sharp point it is called a =claw=. In many cases the nails more or less surround the ends of the digits by which they are borne. Horny =beaks= of epidermal origin occur casing the jaw-bones in several widely distinct groups of animals. Thus among reptiles they are found in Chelonia (tortoises and turtles) as well as in some extinct forms; they occur in all living birds, in _Ornithorhynchus_ among mammals, and in the larvae of many Amphibia.