XX. ON THE DEVELOPMENT OF THE SKELETON OF THE 

 PAIRED FINS OF ELASMOBRANCHII, CONSIDERED IN RE- 

 LATION TO ITS BEARINGS ON THE NATURE OF THE 

 LIMBS OF THE VERTEBRATA 1 . 



(With Plate 33.) 



SOME years ago the study of the development of the soft 

 parts of the fins in several Elasmobranch types, more especially 

 in Torpedo, led me to the conclusion that the vertebrate limbs 

 were remnants of two continuous lateral fins 2 . More or less 

 similar views (which I was not at that time acquainted with) had 

 been previously held by Maclise, Humphrey, and other anato- 

 mists ; these views had not, however, met with much acceptance, 

 and diverge in very important points from those put forward by 

 me. Shortly after the appearance of my paper, J. Thacker pub- 

 lished two interesting memoirs comparing the skeletal parts of 

 the paired and unpaired fins 8 . 



In these memoirs Thacker arrives at conclusions as to the 

 nature of the fins in the main similar to mine, but on entirely 

 independent grounds. He attempts to shew that the structure of 

 the skeleton of the paired fins is essentially the same as that of 

 the unpaired fins, and in this comparison lays special stress on 

 the very simple skeleton of the pelvic fin in the cartilaginous 

 Ganoids, more especially in Acipenser and Polyodon. He points 

 out that the skeleton of the pelvic fin of Polyodon consists essen- 

 tially of a series of nearly isolated rays, which have a strikingly 

 similar arrangement to that of the rays of the skeleton in 



1 From the Proceedings of the Zoological Society of London, 1881. 



2 "Monograph on the Development of Elasmobranch Fishes," pp. 319, 320. 



3 J. K. Thacker, "Median and Paired Fins; a Contribution to the History of 

 the Vertebrate Limbs," Trans, of the Connecticut Acad. Vol. ill. 1877. "Ventral 

 Fins of Ganoids," Trans, of the Connecticut Acad. Vol. iv. 1877. 



