OSTEOLOGY 135 



wards an ' ascending process,' found also in the dinosaurs, 

 which is the equivalent of the intermedium, while the' 

 centrale is represented by a distinct osseous nodule in the 

 adults of the Struthiones (including Dinornis) and tinamous. 

 In Apteryx T.J. Paekee found two osseous centraha. 



The number of toes and phalanges has been already 

 described above. 



The Skull.i — While presenting many characteristic features 

 of its own, the skull in birds shows certain fundamental like- 

 nesses to the skull of the reptilia. As in them, and contrary 

 to what we find in the mammalia, the skull of birds — 



1. Articulates with the spine by a single occipital 

 condyle. 



2. Possesses a quadrate bone for the articulation of the 

 mandible. 



3. The mandible itself is composed of at least a dentary 

 angular and articular portion. 



4. The columella auris is very similar. 



The bird's skull is, however, distinguishable by a number of 

 characters, of which the following are the most important : — 



1. The bones of the cranium are very closely united and 

 fused, this being less marked in the penguins and ratites. 



2. The brain case is large as compared with that of 

 reptiles. 



3. The bones of the skull, as are those of the skeleton in 

 general, are light and contain air spaces. 



4. The columella and the os transversum of the reptiles 

 are absent.^ 



5. There is no distinct postfrontal. 



The bones of the bird's skull, as that of other ver- 

 tebrates, may be distinguished into four categories — 

 (1) those of the cranium ossified from its cartilage ; (2) those 

 of the sense capsules ; (3) those of the visceral arches, and 

 (4) membrane bones connected with the several regions 

 enumerated. 



' H. Maqnus, ' Untersuchungen uber cl. Struktur d. knoohernen Vogelkopfes , 

 Zeitschr. wiss. Zool. xxi. 1871. 



* See however below, under Passerine skull. 



