SUB-KINGDOM I.— VERTEBRATA. 



. CLASS II.—AVE8. 



By R. Bowdleb Sharpe, LL.D., F.L.S., &c. 



Stkuotukally the skeleton of a bird differs very remarkably from that of an 

 ordinary mammal, although it is considerably less unlike that of the egg-laying 

 mammals. In all the features in which the bird's bony framework departs 

 from the mammalian type, it approximates to the reptilian conformation in 

 general features. Still, it must not be supposed that in this respect birds 

 are exactly like any living reptiles. Although in their general structural 

 plan their skeletons are similar, that of the bird has become modified and 

 specialised from the original type, which renders it markedly different from 

 that of either a crocodile or a lizard. One of the chief points in which the 

 skeleton of a bird resembles that of a lizard, and thereby departs from the 

 mammalian type, is the mode in which the skull is articulated to the first 

 joint of the backbone. Whereas in a mammal the articulation takes place by 

 means of a pair of knobs on the hinder part of the skull, which are received 

 into corresponding cups in the first joint of the backbone, in the bird and 

 reptile there is but a single knob, or condyle, fitting into a cup in the first 

 vertebra. Then, again, while in the mammal the lower jaw articulates 

 directly with the solid wall of the true skull, or cranium, in the bird and 

 reptile there is on each side an intervening separate bone, known as the 

 quadrate. A third feature in which the skulls of birds and reptiles re- 

 semble one another, and differ from those of mammals, is the complex 

 structure of each branch of the lower jaw — each branch consisting in the 

 two former groups of several distinct bones, whereas in the latter it is 

 formed of a single bone. 



In order to give strength to the back in flying, the vertebrse of the hinder 

 part of the backbone are fused together into a solid mass, forming the so- 

 called sacrum, which is much more extensive than in mammals. To either 

 side of this sacrum are firmly attached the bones of the pelvis, all of which 

 are very unlike the corresponding bones of mammals, and of which the upper 

 elements, or ilia, are by far the largest. In all living birds the bones of the 

 tail are aborted, and terminate in a triangular piece known as the "plough- 

 share bone." In all birds the ribs are few in number, and in most cases these 

 differ from the mammalian type by the presence of an oblique process on the 

 hinder border. These uncinate processes, as they are called, are met with in 

 some reptiles. 



Perhaps the most extraordiriary feature in the skeleton of an ordinary 

 flying bird is the excessive development of the bones of the breast, which 

 in some cases cover almost the whole of the under surface of the body. By 

 far the largest of these elements is the breast-bone, or sternum, correspond- 

 ing with the element similarly named in mammals. Whereas in flying birds 

 this sternum is strongly keeled, in order to afford a firm basis of origin 

 for the pectoral muscles, in the ostrich and its kindred it is flattened. At the 

 sides the sternum is attached to the true ribs by the intervention of so-called 

 sternal ribs. Superiorly it bears a pair of bones commonly known as cora- 



218 



