124 STRUCTURE AND CLASSIFICATION OF BIRDS 



more than the typical three fingers (see, however, below). 

 The relative length of the arm varies much in birds ; it is 

 longest in the flying gulls, terns, &c., whence the name 

 applied to the former of Longipennes. In the struthious 

 birds it is the shortest, and in many running birds the wing 

 is reduced in length. There is too variation in the relative 

 lengths of the humerus, fore arm, and hand. In the divers, 

 for instance, the upper section of the arm is the longest, in the 

 gulls the fore arm, and in the penguins the hand. The 

 length of the hand in the Macrochires is so great that it 

 equals that of the humerus and fore arm together. The 

 exact reverse is seen in the ratites, where the length of the 

 humerus is greater than that of the rest of the wing. Pye- 

 CRAFT has brought out the interesting fact that during the 

 growth of Opisthocomus the proportions of the different sec- 

 tions of the wing alter. 



A study of the relative lengths of the different parts of 

 the arm shows that a reduction of the wing, and a consequent 

 decay of its powers as an organ of flight, do not invariably 

 follow the same path. In the ostrich the middle segment is 

 the shortest, in the cassowary the hand. 



The length of the humerus, the exact form and degree of 

 development of the deltoid ridge, and of the tubercles for 

 the insertion of muscles, furnish systematists with reliable 

 points for the identification of genera and species. So much 

 of our knowledge of extinct birds depends upon fragments of 

 this and others of the ' long ' bones that the value of slight 

 characters of this description has been thoroughly appraised. 

 A glance, for instance, at Lydekkbr's recently published 

 ' Catalogue of the Fossil Birds in the British Museum ' will 

 reveal the importance of the power of discriminating species 

 by such slight indications, which furnish the student of 

 affinities between families or genera with nothing tangible. 



The radius and the ulna are always separate bones, of 

 which the ulna is the longer ; it is frequently marked on its 

 outer surface with tubercles, to which the quill feathers 

 are attached. The most striking modification of the radius 

 is seen in the Parridse (fig. 70, p. 125), where it is prolonged 



