122 COMPARATIVE ANATOMY 



posteriorly. There is thus a well-developed interorbital septum, 

 and the tropibasic type of skull reaches its extreme. The cranial 

 cavity has become further enlarged at the cost of parts formerly 

 situated extracranially than is the case amongst Reptiles. 

 The bones show a tendency to run together by the obliteration 

 of the sutures originally present between them ; they are usually 

 delicate and spongy ("pneumatic"), thus contrasting greatly with 

 those of Reptiles. 1 Only in the ethmoidal region does the cartilage 

 persist throughout life to any extent. 



The unpaired occipital condyle no longer lies at the posterior 

 boundary of the skull, but becomes relatively shifted forward along 

 the 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: near the base of this, 

 basipterygoid processes, articulating with the pterygoids, may be 

 present. The posterior part of the parasphenoid persists as a 

 large and primarily paired plate, the basitemporal, which underlies 

 the basisphenoid and part of the basioccipital. 



The interorbital septum is thin, as in Lizards, but is more solid 

 and less membranous than in the latter : it becomes ossified 

 anteriorly by a mesethmoid and posteriorly by a presphenoid. 

 Orbitosphenoids and alisphenoids are also developed. The auditory 

 capsules, which are more drawn in to the cranial cavity than in 

 Reptiles, ossify by three centres (prootic, epiotic, and opisthotic) 

 which later become fused with one another and with neighbouring 

 bones, and the relations of the tympanic cavity, auditory fenestrse 

 and columella, including the stapes and extracolumella, 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 

 maxillopalatine apparatus, the delicate palatopterygoid bar, which 

 is always more or less separated from its fellow in the middle line, 

 sliding on the rostrum of the basisphenoid, and so allowing the 

 beak to be raised or lowered to a greater or less extent : thus 

 a complete bony palate is 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 to a greater or less degree. 2 The 



1 It should, however, be remembered that the development of air spaces 

 within the bones of the skull is hinted at in Crocodiles as well as in certain fossil 

 Reptiles. 



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

 palate are important for purposes of classification, as are also the mode of con- 

 nection of the lacrymal with surrounding parts, including the small bones (like 

 those in Lizards) which may be present in the neighbourhood of the lacrymal 

 (supraorbital, infraorbital, lacry mo-palatine). 



