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Show 1890.] FIN-SKELETON OF BATOID FISHES. 679 as in the region of the metapterygium it is always proportionate to the variation in depth of the anterjor face of that cartilage. In Raia, the metapterygium is elongated and rod-like, in Rhina it is expanded and plate-like. Both the meso- and meta-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 rays1; if, in knowledge of this fact, the rays which unite to form that portion of the mesopterygium oi Rhina which represents the free border of that oiRaia be counted, it will be found that the number is greatest in the last-named species, instead of fewest as might have been supposed2. 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 oi Pteroplatea first arrested m y attention in 1887, while preparing a paper which this Society has done m e the honour of printing3 ; homology between the apparent post-mesopterygial cartilage (np.) and the intercalary rays (r.) of Gegenbaur not unnaturally suggested itself at the time, and it occurred to m e 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 m y hands4 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 11 rays ; and the intercalary rays which followed it (r.) were united to form a single plate, except for the lingeriug traces of the demarcation lines between their bases and between the bodies of the second and third of the series. O n the right side (fig. 5) there was present a mesopterygium 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 (figs. 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. Neapel,'vol. v. p. 174 (1884). , . 2 Compare for ex. my figs. 4 or 5 with Gegenbaur s pi. ix. fig. 10. s P. Z. S. 1887, pp. 3-26. * Thanks to my pupil Mr. J. Harrison. |