462 PROF. ST. aEOEGE MIVAET ON THE 



(1) That both the azygos and paired fins are modified axial structures — 



Maclise, and probably Geofi'roy St.-Hilaire, and not certainly Goodsir and 

 Humphrey. 



(2) That neither the azygos nor the paired fins are modified axial structures — 



Cuvier, Huxley. 



(3) That the azygos fins are axial structures, but that the paired ones are not so — 



Gegenbaur at first, and even now if the branchial arches are not considered 

 to be parts of the axial skeleton. 



(4) That the azygos fins are not axial structures, but that the paired fins are so — 



Owen. 



All these writers, and all naturalists as far as I know, have held all the azygos fins to 

 belong to the same skeletal category. It is, however, obviously possible that such 

 uniformity of nature and origin may not obtain, but that the apparent similarity may be 

 due to a kind of homoplasy. 



All these writers have also hitherto, as far as I know, held that, whatever be the nature 

 of the supports of the azygos fins, the skeletal parts of the limbs and paired fins are funda- 

 mentally endoskeletal structures. There is, however, obviously another possibility. 

 Not only may the paired and azygos fins be of fundamentally distinct origin and nature 

 from the axial skeleton, but the hard parts of all these structures (the limbs as well as 

 the azygos fins) may have arisen through centripetal chondrifications or calcifications, 

 and so be genetically exoskeletal. 



This possibility first assumed to my mind some importance when examining the 

 structure of certain Elasmobranch fins ; and the possibility became changed for me into 

 a probability upon reading Mr. Balfour's account of the mode of development of 

 Elasmobranch limbs. 



In 1876, Mr. F. M. Balfour described ' the development of the limbs of Elasmo- 

 branchs as " special developments of a continuous ridge on each side, precisely like the 

 ridges of epiblast which form the rudiments of the unpaired fins," and extending " on 

 a level slightly ventral to that of the dorsal aorta," " from just behind the head to the 

 level of the anus." 



He adds, " If the account just given of this development of the limbs is an accurate 

 record of what really takes place, it is not possible to deny that some light is thrown by 

 it upon the first origin of the vertebrate limbs. The fact can only bear one inter- 

 pretation, viz. tliat the limbs are the remnants of continuous lateral fins.'' 



As to the limb-girdles, supposed by Gegenbaur and Dohm to be modified gill-arches, 

 he tells us, " None of my observations on Elasmobranchs lends any support to these 

 views ; but perhaps, while regarding the limbs as the remains of a continuous fin, it 

 might be permissible to suppose that the pelvic and thoracic girdles are altered 



' Cambridge Journal of Anatomy and Physiology, vol. xi. part i. p. 132. 



