1890.] FIN-SKELETON OF BATOTD FISHES. 679 



as in the region of the metapterygium it is always propor- 

 tionate to the variation in depth of the anterior face of that 

 cartilage. In Raia, the nietapteryo;ium is elongated and rod-like, in 

 Rhina it is expanded and plate-like. Both the meso- and raeta- 

 pterygia of the Plagiostome's fin are known to be identical in origin, 

 and to arise by the coalescence of the bases of originally distinct and 

 parallel rays^ ; if, in knowledge of this fact, the rays which unite to 

 form that portion of the mesopterygium of Rhina which represents the 

 free border of that of Raia be counted, it uill be found that the number 

 is greatest in the last-named species, instead of fewest as might have 

 been supposed". Thus it is seen that the mesopterygium of Raia, 

 so far as it goes, more than embraces that of the Selachoidei, Rhina 

 not excepted ; and, as the anterior of the two supposed mesopterygial 

 plates oi Pteroplatea similarly embraces the characters of them both, 

 in respect to the points at issue, the clue to the morphology of the 

 posterior plate of that fish {np. of figs.) must be sought in something 

 else. 



The facts which I have described for the fin-skeleton of Pteroplatea 

 first arrested my attention in 1887, while preparing a paper which 

 this Society has done me the honour of printing ^ ; homology be- 

 tween the apparent post-mesopterygial cartilage {np.) and the 

 intercalary rays (r.) of Gegenbaur not unnaturally suggested itself 

 at the time, and it occurred to me that if such be the truth, 

 the intercalary rays of Raia might be expected to show signs of 

 fusion to form a basal plate. During the 3-4 years which have 

 elapsed since first I entertained these ideas I have examined 

 some scores of Skate, without having observed any traces of the 

 fusion anticipated. Quite recently, however, there has come into 

 my hands ^ an individual of Raia maculata in which it was realized 

 to an unexpected degree. The mesopterygium of the left side of 

 this fish (fig. 4, ms.) was in relationship peripherally to 1 1 rays ; 

 and the intercalary rays which followed it (r.) were united to form a 

 single plate, except for the liugeriug traces of the demarcation lines 

 between their bases and between the bodies of the second and third 

 of the series. On the right side (fig. .5) there was present a meso- 

 pterygium bearing 12 free rays ; the six intercalary rays had, by 

 the union of their bases, given rise to a single expanded plate, with a 

 smooth inner border and destitute of all traces of demarcation lines. 

 There was thus realized a condition of the basal cartilages of the 

 fin-axis essentially similar to that seen in Pteroplatea (ngs. 1 & 2), 

 except for the numerical disparity in the number of rays involved 

 and for the differences in the mode of articulation upon the 

 shoulder-girdle. That these differences are of secondary and non- 

 morphological significance will, I think, be admitted, on a knowledge 

 of the numerical variation in the rays of the fin-axis for species of the 



1 Cf. P. Z. S. 1887, p. 15, and Dohrn in Mittheilung. a. d. Zoolog. Stat zu 

 Neapel, vol. v. p. 174 (1884). 



2 ComxDare for ex. my figs. 4 or 5 with Gegenbaur's pi. ix. fig. 10. 



3 P. Z. S. 1887, pp. 3-2«. 



* Thanks to my pupil Mr. J. Harrison. 



