32 A HISTORY OF BIRDS 



existence, and forming a smooth, highly polished box whose 

 walls are permeated with air spaces. But in the ripe embryo, 

 or better still, generally, in the nestling, this cranial box is 

 found to be composed of a number of separate pieces correspond- 

 ing closely with those of the reptile. Of these separate ele- 

 ments there is no need to do more than give a very general 

 account of those which have a direct bearing on the question 

 at issue. As in the reptile, the fore-part of the floor of the 

 skull is prolonged into a long parasphenoidal rostrum, at the 

 base of which, in the majority of birds, will be found a pair 

 of " basipterygoid processes ", These are peculiar to reptiles 

 and birds. Largest in the more primitive types, such as 

 the Ostrich tribe, Game-birds and Ducks for example, they 

 have disappeared entirely among some groups, as in the Parrots 

 for example; while in others some members possess vestiges 

 thereof, and others no trace at all, as for instance among the 

 birds of prey and the song-birds. The lower jaw articulates 

 with the skull by means of an anvil-shaped bone, the quadrate, 

 and this again occurs only among reptiles and birds. 



But owing to the relatively enormous increase in the size of 

 the brain, the cranium of the bird differs markedly from that 

 of the reptile, inasmuch as the parietal bones, before fusion, have 

 the whole posterior border interlocked by suture with the supra- 

 occipital and lateral occipital bones, and to these the squamosal 

 is attached, being fitted in, as it were, puzzle fashion, to fill 

 up the gap left where they meet, and serving at the same time 

 to partly cover the bones of the internal ear (111. 8). In the 

 reptile, on the contrary, the brain is relatively small, so that the 

 outer angle of the hinder border of the parietal is produced 

 backwards into a long " flying buttress " to serve for the attach- 

 ment of the squamosal which is thus completely divorced from all 

 participation in the cranial wall. In this particular then the 

 bird really more closely resembles the mammal. The structure 

 of the ear and the lower jaw are other characters which must 

 be cited in this connection. 



Evidence of no less importance is to be had by a study of 

 the developing skull, but this is of an extremely technical 

 character, and need not be cited here. 



Turning now to the skeleton of the trunk, we find that, in 

 the young bird at least, the neck vertebrae bear free ribs — in 



