24 , BIRDS. 



The second digit has two, and sometimes three, phalanges (as in the Swan), 

 and the terminal phalanx is similarly provided with a claw in sundry birds 

 (Swan, Rhea). In the Ostrich both the pollex and the second digit are unguicu- 

 late. 



The third digit possesses one phalanx, besides its ankylosed metacarpal, and is 

 always devoid of a claw. 



Starting from a typical five-fingered hand such as that of a lizard, passing 

 through the Archaeopteryx (the Lizard Bird of the Jurassic period) and the young 

 Duck, it is seen how, by loss of certain carpal bones and fusion of others with 

 the metacarpus, loss of digits v., iv., and part of iii., and fusion of the remaining 

 metacarpals, the typical avian manus seen in the adult Duck is brought about 

 (see diagram). 



Integument and Feathers. 



The exoskeleton of birds consists almost entirely of epidermic structures in 

 the form of horny sheaths, scales, plates, or feathers. No bird possesses dermal 

 ossifications, unless the spurs, which are developed upon the legs and wings of 

 some species, may be regarded as such. 



The Bemiges, or Might-Feathers. 



The feathers of a wing of a bird have a very definite arrangement. They 

 consist of the large quill-feathers {Remiges, or' flight-feathers) and the coverts 

 {Teetrices). 



The Remiges are developed in the wing-membrane {ala membrana), and come 

 into close and definite relation to the bones, forming two well-marked groups : — 



(i.) The cubitals, or " secondaries," are attached to the ulna, and vary greatly 

 in number in difierent groups of birds. 



(ii.) The metacarpo-digitals, or "primaries,"' are attached to the bones of the 

 manus, and are typically eleven in number. Of these the metacarpus bears six 

 (the "metacarpals"), and the phalanges of the digits bear five (the "digitals"), 

 disposed as follows : One (the "addigital") lying upon the phalanx of digit (iii.), 

 and inserted at the metacarpo-phalangeal articulation ; two (the " middigitals ") 

 inserted on the first phalanx of digit (ii.) ; and two (the " predigitals ") inserted 

 on the second phalanx of digit (ii.). The most distal of these (the "remicle") is 

 always rudimentary, and sometimes absent. The bones of digit (i.) always bear 

 a small number of more or less developed quill-feathers, forming the "bastard 

 wing " (ala spuria or alula). 



In many birds (notably, the Wild Duck) there is a development of large feathers 

 lying over the distal half of the humerus, resembling remiges, but really con- 

 tinuous with the series of greater coverts ; these are called " humerals " {Para- 

 pteron). Corresponding with these on the under surface of the wing are the 

 " axillaries " [Hypopteron). 



Modifications of the Metacarpo-digital or Primary Remiges. 



The typical condition of the metacarpo-digital remiges being as described above, 

 the following are the principal modifications : — 



I. The metacarpals may be increased to seven (as in Grebes, Flamingoes, and 

 Storks). 



