ON THE VERTEBRATE SKELETON. 299 
‘from each other by the extension and mutual junction at the median line 
of the occipital and frontal spines. A specimen of this, in a species of 
Cebus, which repeats the common modification of the parts in fishes, is pre- 
served in the museum of the Royal College of Surgeons. The parapophysis 
(s) always commences as an autogenous element by a distinct centre of ossi- 
fication, as shown in the human feetus, fig. 11,8; it speedily coalesces with 
the petrosal, but otherwise retains its individuality in some of the lower mam- 
mals, as e.g. in the echidna (fig. 12,8): or it coalesces with the curtailed 
frontal pleurapophysis 2s, or with the maxillary appendage 27, or with both 
‘these and the pleurapophysis of its own vertebra (38), when the complex 
‘temporal bone’ of anthropotomy is the result. In most mammals the pleur- 
apophysis (38) retains its primitive independency and rib-like form, with 
usually the ‘head’ and ‘tubercle’; but by reason of its arrested growth it 
has been called ‘styloid’ bone or process. Sometimes it is separated from 
the short hemapophysis, 40, by a long ligamentous tract, sometimes it is imme- 
diately articulated with it, or by an intervening piece. The hemal spine, 41, 
is usually small, but thick and alwayssingle. The rudiments of hypobranchial 
elements (46) are retained as diverging appendages of the parieto-hzmal arch 
in all mammals, and have received the special names of ‘ posterior cornua,’ 
or ‘ thyrohyals,’ from their subservient relationship to the larynx. 
In the frontal segment the centrum, 9, and neurapophyses, 10, very early 
coalesce. ‘Two separate osseous centres mark out the body (fig. 26, C, 9), 
and each neurapophysis has two distinct centres (7b. 10, 10), the optic foramina 
(op) being first surrounded by the course of the ossification from these 
points. The superior development of the neurapophysial plates (10), as com- 
pared with those of the parietal vertebra (6), in most mammals, harmonizes 
with the greater development of the prosencephalon ; but the chief bulk of 
this segment of the brain is protected by the expanded spines of the frontal (11) 
and parietal(7) vertebrae, and by the intercalated squamosals (27). And the ap- 
pendicular piece (27) not only usurps some of the functions of the proper cranial 
neurapophyses, but, likewise, the normal office of the frontal pleurapuphysis 
(2s), in the support, viz. of the distal elements of the hzmal arch (29, 32), 
which now articulate directly with 27, in place of 2s as in all oviparous verte- 
brates. The true pleurapophysis of the frontal vertebra (2s) is almost re- 
stricted in the mammalian class to functions in subserviency to the organ 
of hearing, is sometimes swollen into a large bulla ossea, like the parapophyses 
and pleurapophyses of the cervical vertebra of Cobitis ; it is sometimes pro- 
duced into a long auditory tube, and sometimes reduced to the ring supporting 
‘the tympanic membrane. Yet, under all these changes, since its special ho- 
mology is demonstrable with 2s in the bird (fig. 23) and crocodile (tig. 22) as 
well as with the teleologically compound bone, 2s a, b, c, d, in the fish (fig. 6), 
so likewise must its general homology, which is so plainly manifested in 
the fish, be equally recognised. The frontal hemapophysis (fig. 24, 29, 30), 
and the corresponding half of the hemal spine (ib. 32) are connate on each 
side in all mammals, and become confluent at H 111, in most. The hemal 
arch of the frontal segment of the skull, as in other air-breathing vertebrates, 
has no diverging appendage, unless the tympanic otosteals be so regarded, 
an idea which is not borne out by their development. 
The nasal segment (N rv, H rv) is chiefly complicated by the confluence of 
parts of the enormously developed olfactory capsules (1s) in the mammalian 
class, and its typical character is masked by the compression and mutual coa- 
lescence of the neurapophyses, 14. The centrum is usually much elongated, 
as at 13, and socn coalesces with both newrapophyses (14) and nasal capsules 
in the hog. The newral spine (15) is usually divided, but is sometimes single, 
x2 
