xm PHYLUM CHORDATA 409 



(Chauna) and two other species, and, as a rare abnormality, the 

 Common Fowl and Goose, on the first. With these exceptions the 

 hand of the adult bird has lost all the characters of a fore-foot ; 

 but in the young of the Hoatzin (Opisthocomus) claws are present 

 on the first two digits (Fig. 1070, A), which are sufficiently mobile 

 to be used in climbing. Besides the true claws horny spurs are 

 sometimes present on the carpo-metacarpus. 



There is almost every gradation in the proportional length of 

 the hind-limb, from Birds in which nothing but the foot projects 

 beyond the contour feathers, and even the toes may be feathered, 

 to the long-legged Storks and Cranes, in which the distal part of 

 the tibio-tarsus is covered with scales as well as the foot. In 

 aquatic forms a fold of skin or web is stretched between the toes, 

 sometimes including all four digits, as in the Cormorants ; some- 

 times leaving the hallux free, sometimes forming a separate fringe 

 to each digit, as in the Coots and Grebes. As to the toes them- 

 selves, the commonest arrangement is for the hallux to be directed 

 backwards, and Nos. 2, 3, and 4 forwards, but in the Owls No. 4 

 is reversible, i.e., can be turned in either direction, and in the 

 Parrots, Woodpeckers, &c., it, as well as the hallux, is permanently 

 turned backwards. In the Swifts, on the other hand, all four toes 

 turn forwards. The hallux is frequently vestigial or absent, and 

 in the Ostrich No. 4 has also atrophied, producing the characteristic 

 two-toed foot of that Bird. 



Pterylosis. With the exception of the Penguins, most 

 Carinatae have the feathers arranged in distinct feather-tracts or 

 pterylse, separated by apteria or featherless spaces. These are 

 commonly much more distinct than in the Pigeon, and their form 

 and arrangement are of importance in classification (Fig. 1071). In 

 the Ratitse, apteria are usually found only in the young, the adult 

 having a uniform covering of feathers. The Ratitae, also, have 

 nothing more than the merest trace of hooklets on the barbules, so 

 that the barbs do not interlock and the vanes of the feathers are 

 downy or hair-like. In the Penguins the wing-feathers are 

 degenerate and scale-like. 



Many Birds are quite naked when hatched, but in most cases 

 the body is more or less completely covered by a temporary crop 

 of feathers, the nestling -downs, of various forms, but always having 

 a short axis, soft loose barbs, devoid of interlocking apparatus, and, 

 except in the Emu, having no after-shaft (vide p. 410). They are 

 succeeded, as already described, by the permanent feathers. 



Many Birds, such as the Swan, possess down-feathers or plumules 

 throughout life, interspersed among and hidden by the contour 

 feathers or pennce. In the Heron and some other Carinatse are 

 found powder-down patches (Fig. 1071, B, p. d. p, p. d. p'), areas 

 of downs, the ends of which break off and make a fine dust. Semi- 

 plumes are downs with a well-developed axis : filoplumes, as we 



