288 REPORT—1846. 
curved arrows H 11 and Hu. The relations of the superior series of bones 
as neural arches to the optic lobes and cerebrum are even less doubtful than 
in many fishes, by reason of the much smaller degree of independent ossifi- 
cation of the proper capsule of the acoustic labyrinth. Taking, then, the 
bones forming the arch N 11, we find them, viewed from behind, to present 
the general arrangement shown 
in fig. 19. The hinder (basisphe- 
noidal) portion of the bone s and 
9 forms the centrum, and imme- 
diately supports the floor of the 
mesencephalon, or lobe of the 
third ventricle, being’ excavated 
for the pituitary prolongation of 
that cavity: it also sends a pro- 
cess downwards, repeating, like 
the basioccipital, the inferior 
exogenous spine of the centrums 
of the cervical vertebra. The 
bones 6, 6 protecting the sides 
of the mesencephalon, and notch- 
ed for the transmission of the 
trigeminal nerve, manifest the Pie : ; { 
neurapop hysia i characters of the rete ae mesencephalic arch, viewed from behind : 
segment. As accessory func- 
tions they contribute, like the corresponding bones in fishes, to the forma- 
tion of the ear-chamber. They have, however, a little retrograded in posi- 
tion (see fig. 9), resting below, in part, upon the occipital centrum, and sup- 
porting more of the spine of that centrum (3) than of their own (7); which 
is, however, formed of a single bone, and in so far manifests more of the 
normal character of the element completing the neural arch, as its crown or 
key-bone, than does the homologous divided and often divaricated bone in 
fishes. This and other analogous facts show that although the lowest ver- 
tebrate class adheres most, as a whole, to the archetype, yet that it can be 
recognised clearly and unequivocally only by patient study of its modifica- 
tions in all classes: for even the lowest have special exigencies arising out 
of their sphere of existence calling for modifications of the type which are 
not present in other and higher classes. We shall find, indeed, that the con- 
nation of the basi- and pre-sphenoids ceases in mammals, and that they only 
coalesce in that class, being primitively distinct ; so that the second cranial 
centrum (5) may be removed with its neural arch, in the foetal quadruped 
(fig. 24) or human subject (25), without doing violence to nature by the use 
of the saw. The bones s, s, fig. 19, wedged between 6 and 7, here, also, ma- 
nifest more of their parapophysial character than in fishes, inasmuch as they 
are excluded from the inner walls of the cranium, whilst they retain and 
manifest broadly their characters as outstanding processes for muscular at- 
tachment. But, besides affording ligamentous attachment to the hyoid arch 
(29, 40), they articulate largely with the proximal element (1s) of the man- 
dibular arch, whose backward displacement, in comparison with its more 
normal position in the fish’s skull (fig. 5), is as clearly illustrated in the meta- 
morphosis of the anourous batrachia, as is that of the hyoidean or scapular 
arches. 
Referring, then, to the side view of the cranial vertebra of the crocodile 
(fig. 22), we see the hemal arch of the second or parietal vertebra in the 
hyoid (39, 40, 41) retaining so much of its embryonic dimensions as is required 
