THE SKELETON IN BIRDS. THE SKULL. 333 



the lumbar, and some of the thoracic and caudal vertebrae. 

 Sixteen to twenty vertebrae or even more may be included 

 in the compound sacrum, and sometimes the whole of the 

 trunk vertebrae are fused together. In Archaeopteryx however 

 but five vertebrae take part in the formation of the sacrum. 



In Archaeopteryx there are twenty long caudal vertebrae, 

 of which the last sixteen carry a pair of feathers apiece, but 

 in all other birds the tail is short and in the great majority 

 of cases the posterior vertebrae are fused together, forming the 

 pygostyle. In the Ratitae and Tinamidae a pygostyle is rarely 

 or imperfectly developed. In Hesperornis there are twelve 

 caudal vertebrae, six or seven of which are united by their 

 centra only, forming an imperfect pygostyle. 



The free caudal vertebrae are generally amphicoelous. 



THE SKULL. 



The skull of all birds from Archaeopteryx onwards is 

 essentially similar, differing from the skull of reptiles mainly 

 in the extent to which the cranium is arched, and its greater 

 size in proportion to the jaws. 



Most of the bones of the cranium are pneumatic, and all 

 show a marked tendency to fuse together, and have their 

 outlines obliterated by the disappearance of the sutures. The 

 several bones remain longest distinguishable in the Ratitae 

 and to a less extent in the Penguins. The orbits are very 

 large and lie almost entirely in front of the cranium ; they 

 are separated by an interorbital septum which is sometimes, as 

 in Chauna and Scythrops, very complete, sometimes, as in 

 Hornbills and the Common Heron, very slightly developed. 

 As a general rule the sclerotic is cartilaginous. 



The anterior nares are almost always situated far back 

 at the base of the beak near the orbits, but in Apteryx they 

 are placed right at its extremity. In Phororhacos they are 

 placed very high up on the enormous beak and are not separ- 

 ated by any bony partition. 



