682 PROF. G. B. HOWES ON THE PECTORAL [DeC. 2, 



pterygium with that of supernumerary rays. The shoulder-girdle of 

 Tryrjon ' (fig. 9) is, like that of Eaia (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 morpho- 

 logical significance, on analogy to the behaviour of the propterygium 

 in the younger and older stages o{ Pteroplatea, described at the outset 

 {ante, p. 6/7). The structural plan of the pectoral fin of the indi- 

 vidual Trygon before alluded to (fig. 9) is, on the whole, somewhat 

 a simplification of that o^ Pteroplatea (figs. 1, 2) ; and, in view of 

 the condition of its mesopterj^gial area, I regard the posterior of the 

 two plates there present as homologous with the neopterygiura 

 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 forth- 

 coming; baton the whole, and on comparison of Trygon uarnak^, 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. — Gegeubaur originally described the mesopterygium 

 oi Myliobatis aquila (I. c. p. 144) as succeeded by first a single tin-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 perceptible ", 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 



' Three specimens examined. 



^ Indicated as a dotted line in fig. 3. 



