286 REPORT—1846. 3 
skulls of the air-breathing vertebrates. In the young tadpole the thick car- 
tilaginous hyoidean arch * is suspended, as in fishes, from the tympanic pedicle : 
the slender hyoidean arch of the mature frog is suspended from the petrosal 
capsule +. The mandibular arch has, also, receded ; and the scapular areh 
which, at its first appearance, was in close connection with the occiput, further 
retrogrades in the progress of the metamorphosis to the place where we find 
it in the skeleton of the adult frog. 
The argument, therefore, may be summed up as follows. The position of 
the neurapophyses in the dorsal vertebra of chelonians and in the sacral ver- 
tebrze of dinosaurians and birds, -shows that a change of relative position in 
respect of other elements of the same vertebra may be one of the teleological 
modifications to which even the most constant and important elements are 
subject. Instead of viewing such shifted arches as independent individual parts, 
we trace their relation to the stationary elements of the vertebral segments— 
the centrums. Thus, commencing, for example, with the anterior of the 
sacral vertebre of the ostrich, A in fig. 27, we observe that, besides sup- 
porting its own neural arch, it bears a small portion of that of the next ver- 
tebra: the third neural arch (” 1) has encroached further upon the centrum 
of the vertebra in advance ; and thus, in respect to the neural arch ( 2), if 
it were viewed with the centrums, ¢2 and ¢1, upon which it equally rests, 
apart from the rest of the sacrum, it would appear to appertain equally to 
either, and be referable to the one in preference to the other quite gra- 
tuitously. Nevertheless 72 is proved, by the intermediate changes in ante- 
cedent neural arches, to belong actually, and in no merely imaginary or trans- 
cendental sense, to ¢ 2 altogether, and not to the segment of which ¢ 1 is the 
centrum ; and in tracing the modifications of those sacral vertebrae which 
follow ¢ 2, we find 2 4 to have regained nearly the whole of its centrum, ¢ 4, 
and the normal relations of the elements are quite restored in the sueceeding 
vertebra. 
Now let us suppose the habits of the species to have required a more 
extensive displacement of the arch (7 2) and its appendages: if its formal 
characters as a neural arch were still retained beneath the adaptive develop- 
ment superadded to the adaptive dislocation, and if the segments before and 
behind the centrum ¢ 2 were found complete, and that centrum alone wanting 
its neural arch; would the mere degree of modification in respect of relative 
position nullify the conclusion that the shifted arch appertained to such in- 
complete segment, and forbid that restoration to the typical condition, which 
no anatomist, it is presumed, will dispute in the case of m 2, ¢2, fig. 27? No 
anthropotomist hesitates in pronouncing the exact vertebra to which the 
sixth ribs belong in the human skeleton. But, separate that costal arch 
with the two bodies and neural arches of the vertebrae with which it articu- 
lates, and to which of them it belonged would be as questionable as in the 
instance of the displaced neural arch in the bird’s sacrum. The head of each 
rib is applied half to the upper centrum, half ‘to the lower one: the upper 
border of the neck of the rib articulates with the upper neural arch, the tu- 
bercle with the diapophysis of the lower neural arch. Ifa naturalist, not 
conversant with the definitions of human anatomy, were shown this detached 
part of the human skeleton and were pressed to determine the proper centrum 
and neural arch of the hypothetically displaced costal element, the attempt 
might seem to him gratuitous: and to the question, to which of such 
centrums the rib exclusively (as to the pre-existing pattern) belonged ? he 
* Cuvier, Ossem. Foss. v. pt. ii. pl. 24, fig. 23, a. 
+ Ib. fig. 27, a:—an intermediate stage is shown at fig. 25. Dugés and Reichert confirm 
and further illustrate this change of position of the hyoidean arch. 
