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Show 680 PROF. G. B. HOWES ON THE PECTORAL [Dec. 2, genus Raia (cf. Table), and of the variations with age in the articular surfaces oi Pteroplatea already alluded to (ante p. 6/7, figs. 1 & 2). The post-mesopterygial plate of the Trygonid and the free intercalary rays of the Raiidce having been now proved to be homologous, the question arises, which of them is to be regarded as tbe more primitive representative of the other ? That the Batoid type of fin has been derived from a shorter Selachoid one by forward rotation and general enlargement is sufficiently clear, from known facts of development ; and, on comparison of the two types, it might at first sight appear that the post-mesopterygial plate and free rays named above both represent, together with the mesopterygium, the mesopterygium of the Selachoidei, and that the free rays of the Raiidce may have arisen by dismemberment and segmentation of the posterior half of that. Such a possibility is, however, irreconcilable with the fact, already demonstrated, that all the distinguishing features of the mesopterygium of the Selachoidei are realized by that of the Raiidce, apart from the intercalary rays. The last named are related to Parker's "glenoid commissure" (cf. ante, p. 676), and Gegenbaur has sought to correlate (/. c. p. 144) their origin with what he terms the " stretching of the articular region" of the shoulder-girdle. Be their original significance what it may, the facts above described show them to be at present active in the production of a fourth basal cartilage, phylogenetically the youngest of tbe series. The now well-known fact already cited (p. 679) that the basal pterygia of Gegenbaur arise by fusion of the bases of parallel and originally distinct rays, shows the cartilage in question to be serially homologous with the former. I have thus far alluded to it as the post-mesopterygial plate ; as it can no longer be referred to the mesopterygium, I propose to term it, as is consistent with its mode of origin and with Gegenbaur's expressive nomenclature, the neo-pterygium. III.-The Pectoral Fin-Skeleton of Trygon, Urolophus, and Mylio-batis, compared with that o/Raia and Pteroplatea. The pectoral fin-skeleton of Trygon pastinaca has been already described by Gegenbaur (/. c. p. 144) and Haswell (/. c. p. 35). Both observers agree in regarding that basal cartilage which occupies "all the interval between the propterygiun and the metapterygium " as the mesopterygium. Gegenbaur figures in relation to it 13 rays of the axis, five of which reached the pectoral girdle. I have dissected, in all, five individuals of this species, three of Triton uarnak, and two of Urolophus testaceus ; and in all but three of the series the number of these rays exceeds that of Gegenbaur's specimen. In all of them the mesopterygium is comparatively short and plate-like. Both Gegenbaur and Haswell regard it as the homologue of the Selachoid mesopterygium. If, however, the two things be compared under the conditions which I have laid down (ante, p. 679) in dealing with Rhina and Raia, it will be seen that the supposed mesopterygial rays of the Trygons are much more |