18 W. P. PYOKAFT. 



forwards and downwards, thus the articular surface for the squamosal looks distinctly 

 backwards (caudad) and downwards, while the parietal border cephalad of the 

 squamosal forms therewith a sharp angle. Now, taking the Class Aves as a whole, 

 as specialisation proceeds, that is to say, as the cranial cavity increases, the 

 orientation of the roofing bones changes ; so that the supra-occipital, parietal, and 

 frontal regions, instead of lying one in front of the other, come more and more to turn 

 upon themselves so as to lie one upon the other, while the parietal becomes narrower 

 antero-posteriorly, but increases in length laterally. At the same time the squamosal 

 shifts forwards, passing the parietal angle described above, and approaching nearer 

 and nearer to the frontal. In Aptenodytes, where the frontal is almost reached, we 

 have one of the initial stages in this changed relationship of the squamosal. In the 

 majority of the higher Carinatse, indeed, the squamosal actually comes into 

 juxtaposition with a large portion of the frontal bone. A further feature of this 

 specialisation is seen in the fact that in the more primitive skulls the squamosal is 

 entirely superficial, while in the more highly specialised it has come to absorb the 

 underlying tissue of the parietal and so to take a direct part in the protection of the 

 brain. Apparently only in Aptenodytes, among the penguins, does the squamosal 

 pierce the cranial wall in this way, when, partly by a reduction of the upper end of 

 the prootic, and partly by the absorption of the parietal a portion of its inner surface 

 appears within the cerebral cavity. 



We might here observe that in the relation of the squamosal to the cranium Birds 

 resemble the Mammalia rather than Reptilia, a fact which is due to the relatively 

 enormous size of the brain in either case. Among Reptiles the Birds agree much more 

 closely with the Rhynchocephalia than with any other group of living Reptiles. The 

 relations of the palatal bones and the presence of a quadra to-jugal arch afford 

 convincing evidence of this. Thus, in Sphenodon, the quadrato-j ugal articulates, as in 

 birds, with the inferior extremity of the quadrate, immediately above the lower jaw, 

 while in lizards and snakes the rod-like jugal bar extends forwards from the base of 

 the squamosal immediately above the head of the quadrate to join the post-orbital 

 process. 



The Wing of the Embryo. 



The wing of the embryo penguin, if examined at the time when the first traces 

 of bone are beginning to form around the cartilaginous skeleton, will be found to 

 differ remarkably from that of the adult ; and entirely confirms the contention 

 that the paddle of the modern penguin has been derived from a functional flying 

 wing. And this not only because at this early stage the wing agrees, even in detail, 

 with that of typical Carinatse at the same stage, as may be seen in fig. 9, but 

 also because it agrees in all essentials with that of the adult flying bird more 

 closely than at any other later stage of development. But the transformation to the 

 paddle-shaped organ peculiar to the penguins takes place with great rapidity after the 



