67S 



PROF. G. B. HOWES ON THE PECTORAL 



[Dec. 2, 



rig. 6. 



Fig. 4. Horizoutal section of the left pectoral flu-skeleton, with girdle, of an 



exceptional individual of JRaia maculata, ventral aspect. 

 Fig. o. 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 Baia radiata. 

 Fig. 8. The same, in lihinohatus gramdatus. (^ . 



All natural size. 

 Beferences as for figs. 1 to 3 except r, neopterygial (intercalary) rays. 



Table) ; in Bhina they are never more than 3 in number, and they 

 may be reduced to 2 (cf. Gegenbaur, pi. ix. fig. 10). While in 

 liaia 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, or — 

 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 



