354 



SYSTEMATIC ZOOLOGY. 



flying birds possesses a strong ridge or keel (carina) below, 

 to which the muscles of flight are attached. In some 

 flightless birds the keel is lacking. 



The skull is noticeable from the great extent of the 

 fusion of the separate bones; for the single condyle for 

 articulation with the neck and for the suspension of the 

 lower jaw by means of a movable quadrate bone, as in 

 the lizards, snakes, etc. 



The shoulder-girdle consists of scapula, coracoid, and 

 clavicles, the latter noticeable for their union into a V-- 

 shaped 'wish-bone' or furcula. In the wing the reduction 

 in bones of the wrist and hand is remarkable. The bones 

 of the wrist are all united into two, while the three fingers 



FIG. 149. Skull of quail, g, quadrate bone. 



which remain have few joints and are partly united. In 

 the hind limb the fibula is short, but especially noticeable 

 is the great lengthening of two of the ankle-bones, the 

 result being that the heel is elevated some distance from 

 the ground. 



Birds are grouped in three divisions or subclasses, 

 Saururse, Odontornithes, and Ornithurse, the first two of 

 which are extinct; the third contains the ten thousand 

 known species of living forms. 



