5 MR. W. K. PARKER ON THE STRUCTURE AND 
run on along the whole extent of the median keeled bar—the intertrabecula ; in the 
Bird they stop short, leaving a free cartilaginous rostrum, like that of a Shark or 
Skate, which, however, only lasts until it has served as a model on which the huge 
premaxillaries of the Bird are formed. 
In the sides of this hollow cartilaginous structure, near the hind part, the large 
oval auditory capsules (a.s.c., chl.) are seen to have great distinctness; they are, how- 
ever, confluent, with the chondrocranium proper, at various points—above, behind, and 
below, as the sections will show. These are the only sense-capsules displayed in a 
preparation of this kind, for the eye-balls are quite free from the solid cranial 
structure (and are, indeed, outside, in such a view as this), and the left nasal 
labyrinth has been removed. 
Before describing this figure in detail, there is one remark to be made, namely, that 
here we have, clearly shown, the true diagnostic mark of a Mammalian skull. This 
mark is the rupture of the side walls, due to the pressure of the large lateral masses 
of the cerebrum. In front of the auditory capsules there is a large elegantly semi- 
circular opening, the crown of the arch looking upwards and forwards. 
Only the lower half of the wall has thus broken outwards ; this “ fault” forms the 
alisphenoid (a/.s.), whilst the orbitosphenoid (0.s.), the so-called “lesser wing,” is 
many times its size, and is continuous, over the archway, with the cartilage that runs 
on, backwards, into the supraoccipital region (s.0.) 
There is nothing similar to this in that Sautopsidan skull which comes nearest 
to that of the Mammal ;—the skull of the Crocodile (see Trans. Zool. Soc., vol. xi., 
plate 65), whilst in Birds the orbitosphenoids are very small even when they are most 
developed, as in Struthio (see Phil. Trans., 1866, Plate 7), and in that Class the 
alisphenoids almost finish the cranial cavity, being turned inwards towards each other, 
on each side of the back part of the orbital septum. 
I lay especial stress upon this rupture, outwards, of the alisphenoid, and of the fact 
that the nasal roofs utilise the whole of the huge high-crested intertrabecula, because 
these are the most distinctive marks of the Mammalian skull, and they arise out of 
two things in which the Mammal shows its great superiority to even the highest 
Sauropsida, namely, the huge volume of the cerebrum, and the tenfold complexity 
of the nasal labyrinth. 
A third clear diagnostic is seen in this very figure ; this is the peculiar development 
of the antero-inferior part of the oblique auditory capsule, due to the development of 
the coils of the cochlea (figs. 1, 6, 8, chl.). 
So that, at once, correlated with the sudden expansion, so to speak, of the cerebrum, 
we have these new and most important improvements in the organs of smell and of 
hearing. 
At first sight, seeing how large the median bar (intertrabecula) is, with its inter- 
nasal crest (perpendicular ethmoid and septum nasi,—p.e., s.v.), it might be supposed 
