ON THE VERTEBRATE SKELETON. 281 
the hemal spine (22) to the nasal centrum (13), and by the upward develop- 
nt of the processes of 22 which join the neural spine (15). Much modifi- 
cation was to be expected in the segment which terminates the skeleton 
anteriorly ; and yet the typical characters of the neural arch are more com- 
pletely preserved here than at the opposite end of the vertebral column. If 
the bones 4, s, 12, which I recognise as ‘ parapophyses’ in the cranial 
segments 1, I, 111, must be viewed as superadded intercalations for the 
special and characteristic expansion of the neural arches of those segments— 
normal elements, indeed, of the typical vertebra, but with modified connections 
for cranial functions—then the disappearance of their homotypes in the nasal 
segment restores its neural arch (fig. 4.) to the more common condition, and we 
recognise in 13 the centrum, in 14, 14, the newrapophyses, and in 15 the neural 
spine of the nasal vertebra. 
_ But the segment to be complete should exhibit a second arch, inverted ; and 
we find such arch closed or completed by the symphysis of the bones 22, 
fig, 5, and suspended to the sides of the centrum 13 and to the neurapophyses 
145 14, by the bones 20, as the piers or crura of the arch ; these bones being 
connected to the key-bones 22, by the intermediate bones 21. Now, the 
modifications which these elements of the inverted or hzmal arch of the 
nasal vertebra have undergone, are, also, much less than might have been 
anticipated from the extent to which the segments are modified at the oppo- 
site extreme of the endoskeleton. All the normal elements of the hemal 
arch, for example, are. retained: 20 is the pleurapophysis, 21 the hemapo- 
physis, and 22 the hemal spine, in most fishes divided at the middle line, but 
sometimes confluent with its fellow e.g. Diodon. The essential (pleur- 
apophysial) part of 20 extends in many fishes (e. g. percoids) like a short 
straight rib from its articulation with 13 and 14 to the condyle at its opposite 
end to which the hemapophysis 20 is articulated ; but it usually, also, de- 
velopes a process from its hinder margin downwards and backwards, which 
gives attachment to the diverging appendage of the arch Hiv, The de- 
velopment of the other bones of the arch, 21 and 22, outwards, downwards 
and backwards, is still more marked in relation to the protractile and retrac- 
tile movements of the arch in most osseous fishes; and some anatomists, 
influenced by the form and proportions rather than the connections of those 
‘bones, have described them as independent parallel arches: but, as such, 
they must be regarded as being suspended by their apices or key-stones to 
the axis of the skull, and as having their haunches hanging freely downwards 
and outwards—a position the reverse of that of the foregoing inferior arches 
of the skull and of every typical hemal arch. The reduction of that di- 
-vergent development, characteristic of the bones 21 and 22 in fishes, is ef- 
fected in a great degree within the limits of the piscine class: already we 
_ find one of the spurious arches abrogated in the salmonoid fishes by the short- 
ening of 22, and its more direct continuation from 21, which now forms the 
larger part of the upper border of the mouth and supports teeth: the con- 
fluent maxillaries and premaxillaries send down only a single divergent 
process from their point of suspension to the palatine condyle in the plecto- 
hig gnathic fishes; and the consolidation of all the elements of the palato-maxillary 
arch into its normal unity is effected in the lepidosiren*. The palatines (20) 
always form the true bases or suspensory piers of the inverted hemal arch 
at their points of attachment to the prefrontals (14) ; the premaxillaries, 22, 
_ constitute the true apex or crown at their symphysis or point of confluence, 
_ H41v; the approximation of which to the anterior end of the axis of the skull 
is rendered possible, in fishes, by the absence of any air-passage or nasal 
z * Hunterian Lectures, Vertebrata, p. 81, fig. 29. 
_ 1846. U 
