394 



ZOOLOGY 



SECT. 



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 ; 

 sometimes leaving the hallux free, sometimes forming a separate 

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

 themselves, 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 Car- 

 inata3 have the feathers arranged in distinct feather-tracts or 



cd.pl 



FIG. 1000. A, pterylosis of Gypaetos (Bearded Vulture), B, of Ardea (Heron). al. pt, wing- 

 tract ; c. pt, head-tract ; cr. pt, crural tract ; cv. apt. cervical space ; ?. pt, caudal tract ; 

 liu. pt, humeral tract ; lat. apt, lateral space ; p. d. p., p. d. p'. powder down patches ; */>. j>f, 

 spinal tract ; v. apt^ ventral space ; v. pt, ventral tract. 



pterylse, separated by apteria or featherless spaces. These are 

 commonly much more distinct than in the Pigeon (Fig. 1000), and 

 their form and arrangement and of importance in classification. In 

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



