OCR Text |
Show 678 PROF. G. B. HOWES ON THE PECTORAL [Dec. 2, Tig. 6. Fig. 4. Horizontal section of the left pectoral fin-skeleton, with girdle, of an exceptional individual of Raia maculata, ventral aspect. Fig. 5. A similar section of the base of the fin-axis of the opposite side of the same specimen, reversed, for comparison with its fellow. Fig. 6. A similar section of the normal base of the fin-axis in Raia clavata. Fig. 7. The same, in Raia radiata. Fig. 8. The same, in Rhinobatus granulatus. o* • All natural size. References as for figs. 1 to 3 except r, neopterygial (intercalary) rays. Table); in Rhina they are never more than 3 in number, and they may be reduced to 2 (cf. Gegenbaur, pi. ix. fig. 10). While in Raia the mesopterygium bears peripherally from 6 to 12 rays, in Rhina it never bears more than 13. The rays of the axis of the Plagiostome's pectoral fin are well known to be exceedingly variable in the extent to which they coalesce with the mesopterygium, o r - to put the same facts into other words-the degree of outward extension of the mesopterygium is one of the most inconstant characters of the fin in question. It is obvious from this that the number of rays borne upon that cartilage must, to a large extent, increase in proportion to the extension named; but the latter although variable, is not without its constant features, inasmuch |