18 W. P. PYCRAFT. 
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 Carinate, 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 quadrato-jugal arch afford 
convincing evidence of this. Thus, in Sphenodon, the quadrato-jugal 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 Carinate 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 
