FINS or ELASMOBEAifCHS. 463 



remnants of the skeletal parts of some of the gill-arches which have vanished in existing 

 vertebrates." 



This discovery and happy suggestion as to limb-origin made by Mr. Balfour led me 

 to examine the dorsal, ventral, and caudal fins of such Elasmobranchs as I could obtain 

 for the purpose, to see if I could find : — 



(1) Whether any such coalescence of skeletal parts exists in azygos fins as might 



justify the belief that the cartilages of Elasmobranch paired fins had arisen 

 by similar coalescence. 



(2) Whether there is any such solid fixation of the azygos fins to the axial system 



as might harmonize with the fixation of the paired limbs by means of the 

 limb-girdles. 



(3) Whether I could get any light as to the nature of these limb-girdles. 



To take the third point first : I have been long convinced that the shoulder-girdle 

 could not be a branchial arch or form of coalesced branchial arches, as also that the 

 branchial arches could not be, as some have supposed, serial homologues of costal 

 arches ; for the branchial arches are situated within instead of external to the aortic 

 arches, which vessels I took ' to " indicate the line of the pleuro-peritoneal division of 

 the ventral laminae." This conviction has been remarkably justified by Mr. Balfour's 

 recent discovery of the continuation of the pleuro-peritoneal cavity into the head, and 

 externally to these aortic canals ^. 



The branchial arches being then eliminated, there remained the costal arches, the 

 outer cartilages of the gills of Sharks, and other parts external to the body-cavity as 

 possible sources of the shoulder-girdle. It is just these parts which Professor Parker 

 seems disposed to consider as the source of the shoulder-girdle. He tells us (' Mor- 

 phology of the Skull,' p. 343), "The extra branchials of the Dogfish are, at any rate, 

 superficial cartilages related to the branchial arches, and they appear to be homo- 

 logous with the scapulo-coracoid cartilages." Nevertheless a careful consideration 

 of all the leading modifications of piscine structure has ended by convincing me that 

 such could not be the origin of this enigmatical girdle, as to the origin of which I 

 could myself only arrive at negative results. 



With respect to the two former problems, it appeared, and it appears, to me evident 

 that, if the limbs and their hard parts arise in a similar way to that in which the azygos 

 fins and their hard parts arise, they must all probably belong to the same category of 

 peripheral, non-axial structures. 



Moreover, since this is the mode of development of the individual, there arises of 

 course, as Mr. Balfour indicates, an a priori probability that the primeval vertebrate 

 limbs were a pair of continuous lateral folds, serving to balance the body in swimming. 



' Linn. Trans. I. c. and Lessons in Elem. Anat. pp. 222 and 225, fig. 196. 



' Cambridge Journal, vol. xi. part iii. April 1877, page 474. His words are : " It occupies " a " position oc 

 the outer side of the aortic trunk of its arch." 



VOL. X. — PART z. No. 4. — Fehruary Isf, 1879. 3r 



