94 COMPARATIVE ANATOMY 



The unpaired occipital condyle no longer lies at the posterior 

 boundary of the skull, but becomes relatively shifted forward along 

 its base, so that the axis of the latter lies at an angle with that of 

 the vertebral column. 



The basis cranii is formed by a basioccipital and a basisphenoid, 

 from which latter a bony rostrum, the remains of the anterior part 

 of the parasphenoid, extends forwards. The posterior part of the 

 parasphenoid persists as a large plate, the basitemporal, which 

 underlies the basisphenoid and part of the basioccipital. Above 

 the rostrum a small presphenoid is present in the embryo, and 

 orbitosphenoids and alisphenoids are better developed than in 

 Lizards. The auditory capsules ossify by three centres, and the 

 relations of the tympanic cavity, auditory fenestra3, and columella 

 are very similar to those of Reptiles. The two Eustachian tubes 

 open together in the middle line. 



The quadrate is movable upon the skull, as is also the whole 

 rnaxillopalatine apparatus ; the palatopterygoid bar is separated 

 from its fellow in the middle line and slides on the rostrum of the 

 basisphenoid, thus allowing the beak to be raised or lowered to a 

 greater or less extent : a complete bony palate comparable to that 

 of Crocodiles is consequently never present. This mobility of -the 

 upper jaw is most marked in Parrots, in which the frontonasal joint 

 forms a regular hinge. 



The vomers, which may be absent, usually unite with one 

 another, and with the palatines in a greater or less degree. 1 The 

 posterior nostrils are always situated between the vomers and 

 palatines. The maxilla and quadrate are connected by a jugal 

 and a quadratojugal, and a squamosal is present ; small bones may 

 also occur in the neighbourhood of the lachrymal. (For other 

 details, compare Fig. 77.) 



Teeth were present in Jurassic and Cretaceous Birds (Archa30- 

 pteryx, Hesperornis, Ichthyornis), but are no longer developed in 

 existing forms, their place being taken functionally by horny 

 sheaths covering the bones of the jaws, which thus form a beak, 

 much as in Chelonians. 



Several bones are developed in connection with the lower jaw, 

 the relations of which are essentially similar to those seen in 

 Reptiles : they, however, become fused together in the adult, and 

 the two rami of the mandible unite distally by synostosis. 



The visceral skeleton is greatly reduced, though the basihyal 

 and basibranchial which are embedded in the tongue, as well as 

 the first branchial arch persist, and the latter may, as in the 

 Woodpecker, grow out into a pair of very long jointed rods 

 extending far over the skull. 



1 The differences in details as regards the arrangement of the bones of the 

 palate are important for purposes of classification. 



