ON THE VERTEBRATE SKELETON. ‘279 
- The correspondence with the scapular, or occipito-hemal arch, is further 
carried out by the presence of appendages (44) which freely diverge from it, but 
the development of these appendages has not been observed to extend beyond 
_ that second phase, marked by vegetative multiplication of the simple ray, 
directly attached to the arch itself. The lepidosiren offers the simplest con- 
dition of such ‘diverging appendage’ in the single slender bony piece con- 
nected with the element 40*. Cuvier und other ichthyologists cite a series 
of stages of this kind of development of the hyoidean appendage from a si- 
nilar simple beginning up to a 30-fold repetition of the single ray (Zlops); 
and the ‘ branchiostegal’ rays have been found in much greater numbers in 
certain fossil fishes. Like the ‘ pectoral’ rays, they support a duplicature of 
membrane, which plays freely backwards and forwards, reacting upon the 
ambient medium, and forming, in short, a cephalic fin, but with its powers 
so restricted and adjusted, as to propel the water through the branchial cham- 
bers of the fish, instead of driving the fish through the water ; in which latter 
action, indeed, the occipital appendages (pectoral fins)in most osseous fishes 
can and do perform but a very small share. 
If we next proceed to compare the frontal segment, N 111 and H 111, dis- 
‘membered as above described from the parietal vertebra, and, by the separa- 
tion of the sutures, from the bones terminating the skull anteriorly, we shall 
find a neural arch (fig. 3) closely repeating the characters of that of the oc- 
cipital vertebra. The centrum is sometimes represented simply by the forward 
extension of ossification of the basisphenoid (11), which I regard as the ho- 
motype of the ossification of the capsule of the notochord beneath the cen- 
trums of the anterior trunk-vertebre in the silurus ; sometimes, also, of a di- 
stinct superincumbent symmetrical ossicle (9, fig. 5), answering to the rudi- 
mental (central part of the) body of the atlas supported by the inferior bony 
plate, inthesilurus. This more complex condition of the centrum of the frontal 
vertebra is well-seen in the sword-fish. The bones 10, 10, which directly rest 
upono’, when it exists, which defend the sides of the, prosencephalon, and 
which are either grooved by the optic nerves, or have tliose nerves perforating 
the fibro-cartilaginous membrane close to the margin of the bone (10) from 
‘which it is continued, are obviously the newrapophyses. They are, however, 
small; inasmuch as the segment of the brain to which they relate is of inferior 
size in bony fishes: and they are still smaller in comparison with the spine 
-(11) which is enormously expanded, in relation to its accessory functions as 
the chief contributor to and protector of the orbits. The bones 12, wedged 
between the neurapophyses and spine, affording an articular surface to the 
proximal piece of the hemal arch, and developing a transverse process for 
muscular attachments, are the parapophyses. ‘The bones (17) have as little 
essential connection with the typical neural arch above demonstrated, as the 
bones 16, 16” had with the corresponding arch of the parietal vertebra: and 
‘their more peculiar form in relation to the ball which they protect, and their 
variable histological condition in the vertebrate series, have not only prevented 
their ever being mistaken for parts of cranial vertebre, but-have led to the 
opposite extreme of excluding them altogether from the bones of the skull, 
with which they are as much entitled to rank as the petrosal (16) or the 
turbinal (19) ; but always in the category of sense-capsules or ‘ splanchno- 
skeletal’ pieces. 
_ In regard to the inferior arch of the frontal segment, the subdivision of its 
constituent elements, in subserviency to its special functions, is carried to as 
great an extent as in that of the parietal segment. I regard the four over- 
lapping and closely-connected pieces from the upper joint (2s) to the lower 
Set ; * Hunterian Lectures on Vertebrata, p. 79, fig. 27, 37. ny 
