656 MR. F. M. BALFOUR ON THE SKELETON [June 7, 



The imagines that have emerged have nearly all been fine and 

 perfect, a very small percentage indeed of deformed insects coming 

 out ; and as a rule the house is well adapted, in my opinion, for any 

 exotic species and most of the British, the latter emerging much 

 earlier than would be the case in their wild state ; but there is no 

 apparent diminution in size, speaking from imagines obtained from 

 small larvae, as is frequently the case with larvae bred in con- 

 finement. 



June 6, 1881. Wm. Watkins. 



The following papers were read : — 



1. On the Development of the Skeleton of the Paired Fins of 



Elasmobranehiij considered in Relation to its Bearings 



on the Nature of the Limbs of the Vertebrata. By F. 



M. Balfour, F.R.S., F.Z.S., Fellow of Trinity College, 



Cambridge. 



[Received June 2, 1881.] 



(Plates LVII., LVIII.) 



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^ More or less similar views (which I 

 was not at that time acquainted with) had been ])reviously held by 

 Maclise, Humphrey, 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. Tliacker published two interesting memoirs com- 

 paring the skeletal parts of the paired and unpaired fins^. 



In these memoirs Thacker arrives at conclusions as to the nature 

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

 pendent 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 Acipeiiser and Polyodon. He points out that the 

 skeleton of the pelvic fin of Polyodon consists essentially of a series 

 of nearly isolated rays, which have a strikingly similar arrange- 

 ment to that of the rays of the skeleton in many unpaired fins. He 

 sums up his views in the following way^: — 



^ Monograph on the Development of Elasmobranch Fishes, pp. 101, 102. 



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

 of the Vertebrate Limbs," Trans, of the Connecticut Acad. vol. iii. 1877. 



J. K. Thacker, " Ventral Fins of Ganoids," Trans, of the Connecticut Acad, 

 vol. iv. 1877. 



3 Loc. cit. p. 298. 



