Three kinds of feathers are found, viz. (_a_) _pennae_ including quills and coverts, (_b_) down feathers or _plumulae_, and (_c_) _filoplumes_ which are rudimentary feathers. The structure of the different kinds of feathers is described on pp. 303-306. Sometimes a fourth class of feathers, the _semiplumae_, is recognised. They have the stems of pennae, and the downy barbs and barbules of plumulae. In most birds the pennae are not uniformly distributed over the whole surface of the body, but are confined to certain tracts, the =pterylae=; while the intervening spaces or =apteria= are either bare or covered only with down feathers. In some birds, however, such as the Ratitae and the Penguins, pennae are evenly distributed over the whole body. In many birds the calamus or quill bears two vexilla or vanes, the second of which, called the =aftershaft= or =hyporachis=, is generally much the smaller, and is attached to the under surface of the main vexillum. In the Moas, Emeu and Cassowary the two vexilla in the adult bird are nearly equal in size; though in the nestling Emeu one is much longer than the other. The aftershaft is very small in most Passeres and gallinaceous birds, but is comparatively large in Parrots, Gulls, Herons and most birds of prey. It is absent or extremely small in the Ostrich, _Apteryx_, _Rhea_, Pigeons, Owls, Anseres, and others. The quill feathers include two groups, the =remiges= or wing quills, and the =rectrices= or tail quills. In most birds the primary remiges, or those which are attached to the bones of the manus, are ten or eleven in number, and are set in grooves in the bones, being firmly attached to them. In the Ostrich however the primaries are little specialised in character and are as many as sixteen in number. They are also less definitely attached to the bones; as their ends do not lie in grooves in the bones, but project beyond them. The secondary quills or those attached to the ulna vary much in number according to the length of the bone. The large dark quills in the wings of Cassowaries are the secondaries. The wing of Penguins is very little differentiated. It is covered at the margin by overlapping scales which gradually merge into scale-like feathers at the proximal end. The wing of the Penguin has nothing comparable to the remiges of other birds. In some birds, such as Herons (_Ardea_), there occur in places plumulae of a peculiar kind, which grow persistently and whose summits break off into fine powder as fast as they are formed. These feathers are known as _powder-down_ feathers. They occur also in some Parrots and are then scattered indiscriminately all over the body. Other exoskeletal structures besides feathers are commonly well developed. Thus the extremities of the jaws are sheathed in horny =beaks= whose form varies enormously according to the special mode of life.