ON THE VERTEBRATE SKELETON. 297 
of the hemal arch of the nasal vertebra with that of the frontal vertebra, 
and consisting of the two styliform ossicles (26 and 27) which extend from the 
hemapophysis, 21, 21’, to the pleurapophysis, 28 : the essential relationship of 
the compound ray, 26 and 27, with the nasal vertebra, is indicated by their 
becoming confluent with its hemapophysis (at 22”), whilst they always main- 
tain an arthrodial articulation with the pleurapophysis (2s) of the succeeding 
vertebra. 
The bones of the splanchno-skeleton intercalated with the segments of the 
endoskeleton in the bird’s skull are the petrosal (16), between the neural 
arches of the occipital and parietal vertebra, early coalescing with the ele- 
ments of those vertebre with which it comes in contact: the sclerotals (17), 
interposed between the frontal and nasal neural arches: and the thyro-hyals 
(a7), retained in connection with the debris of the hemal arch of the parietal 
vertebra, H u. The olfactory capsule remains cartilaginous. The dermal 
bone (73) is well-developed and constant: a second supraorbital dermal bone 
is occasionally present. All the endoskeletal bones manifest, under every 
adaptive modification, the segmental arrangement, and it is difficult to con- 
template the repetition of the arrangement of the cranial bones around the 
primary segments of the encephalon in the series of arches closed respectively 
by the bones N 1, N 11, N 111, N tv, together with that of the corresponding 
number of arches closed below, at H rv, H 111, H 11 and H1, without a con- 
viction that the type illustrated in fig. 15 is that upon which these seg- 
ments of the skull have been constructed. ‘This conclusion might seem 
forced, in respect to the occipital vertebra, were its displaced hemal arch 
and appendages to be viewed without reference to their relative position and 
connections in the lower vertebrate classes; but it will be confirmed and 
shown to be agreeable to nature and to the recognised kinds and grades of 
modification to which the elements of one and the same vertebra are subject, 
by observing in the young bird the distinct pleurapophysial elements of those 
cervical vertebrae, beyond which the corresponding elements of the occiput 
have retrograded, in obedience to the functions which the hemal arch of 
that vertebra and its appendages are destined to perform in the feathered 
class. 
Mammals.—ltf the foregoing views of the general homology of the bones 
of the skull be agreeable to their essential nature, we should expect that the 
new and additional modifications, in the mammalian class, which tend to 
obscure those relations would be seated in the appendages and peripheral 
elements of the endoskeletal segments, or in the capsules and appendages of 
the special-organs of sense. 
I have selected with a view to testing such anticipation the skull of a young 
pachyderm*, and, after successively disarticulating the segments in the order 
in which they have been previously described, I have given a side view of 
them arranged in correspondence with the figures 23, 22, and 5. (Fig. 24.) 
The neural arch of the occipital vertebra, N 1, agrees with that of the bird and 
crocodile in the coalescence of the parapophysis, 4, with the newrapophysis, 
2; but the process, 4, now descends from the lower part of the arch, and, 
as in many other mammals, is of great length. An articular condyle is also 
developed from each neurapophysis which articulates with the concave an- 
terior zygapophysis of the atlas, and is the homotype of the posterior zyga- 
pophysis in the trunk-vertebre. The centrum (1) is reduced, like that of 
_ the atlas, to a compressed plate, and its hinder articular surface is not more 
is 
* The skull of the ruminant is perhaps still better adapted to demonstrate the vertebral 
relations of the cranial bones: that of the sheep is the subject of the diagram for this pur- 
pose in the concluding volume of my ‘ Hunterian Lectures.’ 
1846. x 
