On the Development of the Skeleton of the Paired 

 Fins of Elasmobeanchii, considered in relation to 

 its hearings on the Nature of the Limbs of the 

 Vertebrata. By F. M. Balfour, F.R.S., F.Z.S., 

 Fellow of Trinity College, Cambridge. 



(With Plates IV and V.) 



Some years ago the study of the development of the soft 

 parts of the fins in several Elasmobranch types, more espe- 

 cially in Toipedo, led me to the conclusion that the verte- 

 brate limbs were remnants of two continuous lateral fins.^ 

 More or less similar views (which I was not at that time 

 acquainted with) had been previously held by Maclise, Hum- 

 phry, and other anatomists ; 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. Thaclier published two interesting 

 memoirs comparing the skeletal parts of the paired and un- 

 paired fins.^ 



In these memoirs Thacher arrives at conclusions as to the 

 nature of tne fins in the main similar to mine, but on en- 

 tirely independent grounds. He attempts to show that the 

 structure of the skeleton of 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 

 oi Polyodon consists essentially of a series of nearly isolated 

 rays, which have a strikingly similar arrangement to that of 



' * Monograph on the Development of Elasmobranch Fishes,' pp. 101, 

 102. 



- J. K. Thacher, "Median and Paired Fins; a Contribution to the 

 History of the Vertebrate Limbs," ' Trans, of the Connecticut Acad.,' 

 vol. iii, 1877. 



J. K. Thacher, "Ventral Fins of Ganoids," 'Trans, of the Connecticut 

 Acad.,' vol. iv, 1877. 



