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Show 682 PROF. G. B. HOWES ON THE PECTORAL [Dec. 2, pterygium with that of supernumerary rays. The shoulder-girdle of Trygon (fig. 9) is, like that of Raia (figs. 4, 6), discontinuous laterally, between the anterior and middle glenoid condyles. It will be observed that the base of the propterygium in Trygon, unlike that of Pteroplatea (figs. 1, 2), is simple and destitute of a second articulation; and this fact, which might readily account for the differences in the limb-girdles, would appear to be of no morphological significance, on analogy to the behaviour of the propterygium in the younger and older stages oi Pteroplatea, described at the outset (ante, p. 677). The structural plan of the pectoral fin of the individual Trygon before alluded to (fig. 9) is, on the whole, somewhat a simplification of that of Pteroplatea (figs. 1, 2) ; and, in view of the condition of its mesopterygial area, I regard the posterior of the two plates there present as homologous with tbe neopterygium herein described. And I submit that, with this, the basal fin-skeleton of Trygon and Urolophus is brought into complete harmony with that of their ally Pteroplatea, and that its axial portion represents a confluence of those parts which, in the latter and in the Raiidce, are differentiated to form the mesopterygium and neopterygium as I have sought to define them. Examination of the Table which I append will show that in Trygon pastinaca there are indications of a numerical increase of the rays of the fin-axis, with age ; and, as the neopterygium is present as a distinct plate only in the oldest example, the possibility that that may be formed late, in connection with the said numerical increase, must not be overlooked. Proof that such is the case is not forthcoming ; but on the whole, and on comparison of Trygon uarnak1, I am inclined to regard the numerical differences alluded to as of the nature of individual variations. IV.-The Pectoral Fin-Skeleton of Myliobatis and of the Torpedinidse. Myliobatis.- Gegenbaur originally described the mesopterygium oi Myliobatis aquila (I. c. p. 144) as succeeded by first a single fin-ray and then by a couple of plates carrying respectively four and five to six rays each. Concerning the relationships of these to the limb-girdle he does not furnish details. I have examined three individuals of this species, and, in all, the rays of the fin-axis were almost entirely confluent with the girdle adjacent (fig. 3), so much so that I was at first inclined to doubt the existence of basal pterygia in that region. On closer examination, however, distinct traces of a line of fusion between the latter and their girdle were found to be perceptible2, and fragments of the pterygia were encountered, in the form of isolated plates such as that shown at * in the fig. In three of the fins dissected there was present a well-defined demarcation line, at about the 1 Three specimens examined. 2 Indicated as a dotted line in fig. 3. |
