ON THE VERTEBRATE SKELETON. 315 
animals, In the fish, in fact,—by reason of the parietal parapophyses (8, s) 
being subject to the same variation in their relative position to the other 
elements, which has been illustrated in respect of the neural spine in the 
epencephalic arch of the dog and sheep,—the mesencephalic arch is com- 
posed of seven pieces, or, including the interposed supraoccipital, of not less 
than eight bones. Yet even here we clearly and easily trace the kind and 
degree of modification to which the fundamental plan of the neural arch 
has been subject. The archetype is nowise obliterated: the general homo- 
logies of the modified elements are not less recognisable than their special 
homologies. The centrum and neurapophyses are the steadiest elements: 
the spine is not only subject to great diversity of size and shape, but to some 
variety of position, and, moreover, to be either single or bifid: the parapo- 
physes have less range of variety in point of dimensions, but may be more 
or less interposed between spine and neurapophyses, or may become con- 
fluent with either element. Thus the epencephalic arch of the crocodile 
(fig.18) differs essentially, in a Cuvierian sense, from that of the tortoise or the 
fish (fig. 1), because it is composed of four pieces in the first and of six 
pieces in the latter; the difference of composition merely depending, how- 
ever, on the more exterior position and connation of the parapophyses, 4, 4, in 
the crocodile. 
The independency of the parietal and frontal bones is next urged by 
Cuvier as militating against the idea that they complete a vertebral arch 
formed respectively by the alisphenoids and orbitosphenoids as the piers or 
haunches: and the more so, inasmuch as they are separated from those bones 
in some animals by the intercalation of the squamosals*. By parity of reason 
we must reject the general homology of the neural arch and spine of the 
atlas in the Ephippus and some other fishes, because that part of the verte- 
bra is not only distinct, but uplifted and removed from the piers or base of 
the arch by the intercalation of the articular processes of the neural arches 
of the occiput and axis. According to Cuvier such separated atlantal arch 
must. be regarded as a new bone, and the centrum ought therefore equally 
to be viewed as ‘une piéce particuliére qui a une destination particuliére ’: 
but the general homology of vertebral elements may be determined not only 
by their relations to their own segment, but by those which they maintain 
with their less modified homotypes in contiguous segments. 
The centrum of the atlas in the Ephippus directly sustains other neur- 
apophyses than its own, and so far has a new or particular function ; but, 
since it continues to unite the centrum of the axis with that of the occiput, 
we still regard it as their homotype, and as standing in the relation of the 
centrum to its uplifted and shifted neurapophyses. So, likewise, although 
these elements now aid in strengthening the joint between the zygapophyses 
of the neural arches of the occiput and axis, and thus perform a new and 
very peculiar function, their relation to these and other neural arches in the 
series of vertebre renders it impossible to overlook the serial homology of 
the separated ‘ laminz’ of the atlas and that of its spine with the other and 
larger vertebral laminz and spines. 
* “ Tans tous les cas, on ne pourrait regarder cette vertébre comme annulaire, ni supposer 
ane Jes pariétaux en forment le complément ; d’une part, ce serait une composition différente 
e celle des autres vertébres, puisque l’anneau serait formé de cinque piéces et méme de 
six, en comptart l’inter-pariétal; de autre, il arrive dans plusieurs animaux que les ailes 
temporales du sphénoide n’atteignent pas au pariétal, parceque le temporal va toucher au 
dessus d’elles, soit au frontal soit au sphénoide antérieur. Ainsi les pariétaux sont des 
piéces indépendantes du sphénoide postérieur, des piéces particuliéres qui ont une desti- 
nation particuliére, celle de servir de bouclier 4 la partie moyenne et postérieure des hémi- 
_ sphéres, tout comme les grandes ailes ont celle de servir de support aux lobes moyens dans 
a we 
lesquels ces hémisphéres se terminent vers le bas.”—I. e. p. 713. 
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