of the Pelvic Plexus in Acanthias vulgaris. 23 



is 29. This he takes as evidence that the girdle migrates caudally 

 between the stages of 32 '0 and 40 '0 mm. and then rostrally again 

 between the stage of 40 -0 mm. and the adult ( (11), pp. 592, 593). To 

 base such an important deduction on four specimens, whilst com- 

 pletely ignoring the possibility of his 400 mm. stage being merely a 

 variation, seems greatly to weaken the case for ontogenetic migra- 

 tion of the pelvic fin in Spinax niger. Braus' argument rests on the 

 assumption that no variations occur in the position of the girdle-piercing 

 nerve in this species. Though I have never had an opportunity of 

 examining Spinax, I have been able to make observations on a number 

 of specimens of each of the following species *Mustelus vulgaris, M. 

 Icfvis, Acanthias vulgaris, Galeus canis, Carcharias glaucus, Scyllium cani- 

 cula, and Scyllium catulut*. In all these species there occurred variations 

 of several metameres in the position of the girdle. Until, therefore, it 

 has been definitely shown that Spinax is constant with regard to the 

 position of its pelvic girdle, any deductions based on the assumption 

 that no variations occur in this species are to be accepted with the 

 greatest caution.! 



General Remarks. 



As has already been stated, the idea with which this investigation 

 was commenced lay in ascertaining the amount of variation shown on 

 certain points in a given species from a given locality, and to discover 

 how far these were explicable on either of the two rival theories the 

 side-fold theory and the migration theory of the origin of limbs.J 



It has been seen that certain of the facts are not in accord with the 

 side-fold theory, without considerable strain on the imagination, though 

 all are explicable on the theory of migration. Consequently the 

 general tenour of the paper is on the side of limb migration. 



* Twenty-three specimens obtained from Plymouth show a range of variation 

 with regard to the girdle-piercing nerve, post-girdle nerves, and whole vertebra: 

 amounting to four metameres. This species is the common form in the Channel. 

 I have never been able to procure M, Icevis, and according to Day ((2), vol. 3, 

 p. 295) it does not occur here. At Naples, M. Icevis is the common form, 

 M. vulgaris being much scarcer. I have previously given evidence to show that 

 M, vulgaris is a more stable form than M. Icevis in the Mediterranean ((12), 

 p. 342). We may look upon this as evidence for the greater variability of the more 

 abundant form, or possibly we might regard the M. vulgaris of the Mediterranean 

 as having acquired greater stability by the splitting off of a variable factor as 

 M. Icevis. This, of course, IB mere speculation. 



t Since the above was written I have had the opportunity of examining a 

 number of specimens of Spinax niger on this point, and find that here, as in all other 

 Elasmobranchs examined, there exist variations in the position of the pelvic girdle 

 amounting to several metameres. 



t This paper is not concerned with the more precise origin of the free limb, 

 whether it has been derived from gill-arch rays, according to Gegenbaur, or from 

 external gills, as recently suggested by Graham Kerr (' Proc. Cambridge Phil. 

 Soc.,' 1899). 



