ISf/O.] FIN-SKELETON OF BATOID FISHES. (\77 



girdle, its two posterior rays being alone in fibrous connection there- 

 with. In the older of the two specimens which 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 (w^j.) 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 super- 

 ficial 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 oi 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 de- 

 scribed by Gegenbaur (/. c. p. 144) in " Raia ? «/?." 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. (385, 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 Ruia 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 usually 

 4 or 5 in number, but they may be reduced to 3 {cf fig. 7 and 



' Cf. Gegenbaur, ' Untersuchung. z. vergleich. Anat. d, Wirbclth.' Heft 2, 

 pi. ix. fig. 10 (1865). 



