OCR Text |
Show 18f)0.] FIN-SKELETON OF BATOID FISHES. 677 girdle, its two posterior rays being alone in fibrous connection In tbe older of the two specimens winch I have examined, the two basal plates of the fin-axis bear (ms., np., fig. 2) essentially the same relationships to the girdle. The propterygium is in a merely fibrous connection with the anterior mesopterygial plate (ms.), and the synovial articulation between the two is here unrecognizable. The former (pp.) bears postero-internally a shallow facet for the reception of a corresponding condyle of the adjacent limb-girdle ; the supposed mesopterygial plate (ms.), apparently the more free to move upon its base, takes on a synovial articulation with the limb-girdle, in common with its fellow (np.) of the same side. The differences between these two pairs of fins are not a little remarkable and unexpected, and that they are not sexual is clear from both specimens having been males (cf. Table on p. 685). I know of no parallel for them elsewhere ; and that, as effecting the pro- and meso-pterygia, they are a corollary of each other, it seems to me in the highest degree probable. II.-The Pectoral Fin-Skeleton of Pteroplatea, compared with that of the Raiidse and of the Selachoidei. The pectoral fin-skeleton of Pteroplatea differs most markedly from that of any known Plagiostome thus far described. On superficial examination, its two supposed mesopterygial plates might appear to correspond to the well-known single one of the Selachoidei, and to represent therefore a subdivision of that structure as it exists in the genus Rhina (Squatina). The entire mesopterygium of Rhina bears, however, but some 10-12 rays, whereas more than double that number are present in Pteroplatea ; in Rhina but 2-3 of these rays reach the shoulder-girdle \ while in Pteroplatea they either all, or all but two or three, do so. From this it is clear that the conditions of the supposed mesopterygium in Pteroplatea are such as the known facts of anatomy of the Selachoid fin, in even its most expanded form, are inadequate to explain. On turning to the Batoidei, it is seen that the posterior moiety of the axis of the fin is supported by a greater or smaller number of free rays (figs. 6, 7, r.) disposed serially with those forming the mesopterygium (ms.) and intercalated between it and the head of the metapterygium (mt.). These intercalary rays were first described by Gegenbaur (I. c. p. 144) in " Raia ? sp." as four or five in number. I find them to be more numerous and usually from six to seven in number in the commoner species, R. radiata excepted (cf. Table on p. 685, and fig. 7), and I think it tolerably certain that Gegenbaur's specimen (/. c. pi. ix. fig. 13) was of the latter species. A careful comparison of the mesopterygium of Raia and Rhina brings into prominence some considerations of importance in the present enquiry. In Raia, the rays of the mesopterygium which reach the shoulder-girdle and furnish the articular facet are usuallv 4 or 5 in number, but they may be reduced to 3 (cf. fig. 7 and 1 Cf. Gegenbaur, ' Untersuchung. z. vergleich. Anat. d. Wirbelth.' Heft 2, pi. ix. fig. 10 (1865). |