ORIGIN AND CHARACTERISTICS 31 



various classes varies considerably, not only 

 amongst species but even in individuals — a 

 fact which renders them of no value from a 

 taxonomic point of view. We now come to a 

 consideration of that pa^t of the skeleton 

 technically called the sternum, or in more 

 familiar language the breast-bone. This very 

 important part of the avine frame is joined to 

 the vertebral column, as we have already seen, 

 by the thoracic ribs, whilst its anterior margin 

 supports the coracoids, or strong bones con- 

 necting it with the scapulae or shoulder-blades 

 and the clavicles or collar-bones, the three 

 pairs of bones combined forming the Pectoral 

 Arch or shoulder girdle. The last-named are 

 generally united or fused at the ventral end, 

 into a V-shaped Furcula, much better known 

 as the *' wish-bone " or ^' merry-thought." The 

 furcula is subject to considerable modification 

 in certain birds, notably in the Crane, the Swan, 

 and the Frigate Bird ; in other birds the clavicles 

 are degenerated, the dorsal portion alone being 

 retained, the ventral end being reduced to a 

 ligament. In some birds (the Parrots for in- 

 stance) the clavicles are absent. In many 

 instances the furcula is ossified with the anterior 

 portion of the keel of the sternum ; whilst in 



