2S6 report— 1846. 



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 f« The mandibular arch has, also, receded ; and the scapular arch 

 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 vertebrae of chelonians and in the sacral ver- 

 tebrae 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 vertebrae 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 (n 1) has encroached further upon the centrum 

 of the vertebra in advance ; and thus, in respect to the neural arch (n 2), if 

 it were viewed with the centrums, a. and ei, 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 n% is proved, by the intermediate changes in ante- 

 cedent neural arches, to belong actually, and in no merely imaginary or trans- 

 cendental sense, to c 2 altogether, and not to the segment of which c 1 is the 

 centrum ; and in tracing the modifications of those sacral vertebras which 

 follow c 2, we find n 4 to have regained nearly the whole of its centrum, c 4, 

 and the normal relations of the elements are quite restored in the succeeding 

 vertebra. 



Now let us suppose the habits of the species to have required a more 

 extensive displacement of the arch (n 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 c 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 n 2, ci, 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. If a 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. pi. 24, fig. 23, a. 



f lb. fig. 27, a: — an intermediate stage is shown at fig. 25. Duges and Reicbert confirm 

 and further illustrate this change of position of the hyoidean arch. 



