670 SKELETON OF FINS OF ELASMOBRANCHS. [JuUC 7, 



to the establishment of the Elasmobraiieh fin, but in exactly the 

 reverse order. 



There is one statement of DavidofF's which I cannot allow to pass 

 without challenge. In comparing the skeletons of the paired and 

 unpaired fins he is anxious to prove that the former are in- 

 dependent of the axial skeleton in their origin and that the latter 

 have been segmented from the axial skeleton, and thus to show 

 that an homology between the two is impossible. In support of 

 his view he states' that he has satisfied himself, from embryos of 

 Acanthias and Scyllium, that the rays of the unpaired fins are un- 

 doubtedly products of the segmentation of the dorsal and ventral 

 spinous processes. 



This statement is wholly unintelligible to me. From my exami- 

 nation of the development of the first dorsal and the anal fins of 

 Scyllium I find that their rays develop at a considerable distance 

 from, and quite independently of, the neural and haemal arches, and 

 that they are at an early stage of development distinctly in a more 

 advanced state of histological differentiation than the neural and 

 haemal arches of the same region. I have also found exactly the 

 same in the embryos of Lepidosteus. 



1 have, in fact, no doubt that the skeleton of both the paired and 

 the unpaired fins of Elasmobranchs and Lepidosteus is in its develop- 

 ment independent of the axial skeleton. The phylogenetic mode of 

 origin of the skeleton both of the paired and of the unpaired fins 

 cannot, however, be made out without further investigation. 



EXPLANATION OF THE PLATES. 



Plate LVII. 



Fig. 1. Transverse section through the pelvic fin of an embryo of Scyllium be- 

 longing to stage P^, magnified 50 diameters, hp, basipterygium; br, 

 fin ray ; m, muscle ; hf, horny fibres supporting the peripheral part of 

 the fin. 



2. Pelvic fin of a very young female embryo of SoylUum stellare, magnified 



16 diameters, bp, basipterygium ; pu, pubic process of pelvic girdle 

 (cut across below) ; ?7, iliac process of pelvic girdle ; fo, foramen. 



3. Pelvic fin of a young male embryo of Scyllium stellare, magnified 16 



diameters. bp), basipterygium ; mo, process of basipterygium con- 

 tinued into clasper : il, iliac process of pelvic girdle ; pii, pubic section 

 of pelvic girdle. 



4. Transverse section through the ventral part of the trunk of an embryo 



Scyllium of stage P, in the region of the pectoral fins, to show how 

 the fins are attached to the body, magnified 18 diameters, br, cartila- 

 ginous fin-ray ; bp, basipterygium ; m, muscle of fin ; m p, muscle-plate. 



5. Transverse section through the ventral part of the trunk of an embryo 



Scyllium of stage P, in the region of the pelvic fin, on the same scale 

 as fig. 4. bp, basipterygium ; br, cartilaginous fin-rays ; m, muscle of 

 the fins; mpi, muscle-plate. 



Plate LVIII. 

 Fig. 6. Pectoral fin of an embryo of Scyllium canicula, of a stage between O 



^ Loo. cit. p. 514. 



2 I employ here the same letters to indicate the stages as in my Monograph 

 on Elasmobranch Fishes. 



