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Show 656 MR. F. M. BALFOUR ON THE SKELETON [J 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 m y 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 confinement. June 6,1881. WM. W A T K I N S . The following papers were read :- 1. O n the Development of the Skeleton of the Paired Fins of Elasmobranchii, considered in Relation to its Bearings on the Nature of the Limbs of the Vertebrata. By F. M . B A L F O U R , F.R.S., F.Z.S., Fellow of Trinity College, Cambridge. [Received June 2, 1881.] (Plates LVIL, 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 m e to the conclusion that the vertebrate limbs were remnants of two continuous lateral fins1. More or less similar views (which I was not at that time acquainted with) had been previously 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 m y paper, J. Thacker published two interesting memoirs comparing the skeletal parts of the paired and unpaired fins2. 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 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 of Polyodon consists essentially of a series of nearly isolated rays, which have a strikingly similar arrangement to that of the rays of the skeleton in many unpaired fins. He sums up his views in the following way3:- 1 Monograph on tbe Development of Elasmobranch Fishes, pp. 101, 102. 2 J. K". Thacker, " Median and Paired Fins; a Contribution to tbe History of tbe Vertebrate Limbs," Trans, of tbe 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. |