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Show 686 PROF. G. B. H O W E S O N T H E PECTORAL [Dec. 2, character; they also show the basal pterygia to have (cf. ante, p. 680) by the fusion of parallel rays. Th_3 being so, the fin of the Rhinobatidce, while clearly specialized as regards the first proposition, is less modified than that of all other Batoid fishes in respect to its feeble expansion. Until we know more than at present concerning the manner of multiplication of fin-rays with bodily elongation and growth, we must regard the presence of free rays in the position of those intercalated between the meso- and meta-pterygia of these fishes as none other than a primitive character; and, in respect to this, the Rhinobatidce would appear to exhibit a more lowly structural feature than the, for the most part, less modified Selachoidei. Whether they may not have reverted to it, it is at present impossible to say ; but I regard the matter as the more interesting in that Edinger has attempted to show1 the prosencephalon of tbe Skates to be more lowly than that of the Sharks, and that I have found2 the primitively continuous dorsal mesentery of the alimentary viscera to be alone retained by the Torpedo Hypnos subnigrum among living Plagio-stomes. It raises, among other things, the question whether this type of fin-skeleton, which Huxley3 would apparently associate with his " multibasal" one, may not represent the (admittedly modified) survivor of a type more primitive than that of the living Selachoidei, rather than a culminating term in a series of changes which he has pictured (/. c. p. 52) as of the nature of an expansion with interpolation of postaxial rays, under a shortening up of the supposed " archipterygium." For the greater part of the material upon which this investigation is based, I stand indebted to the late Dr. F. Day, and, through my honoured master Prof. Huxley, to Mr. Ramsay (of the Sydney Museum), to whom we owe the possession of the Australian forms. M y thanks are also due to m y friend Mr. G. A. Boulenger for a continuance of that assistance and advice extended to me on former occasions, and to m y friend and former pupil Dr. J. Beard for the reference to Edinger's work cited. P.S.-Since this paper was written Mr. Boulenger has directed my attention to a short paper recently published by O. JaekeP, in which the author arrives at the conclusion that the Batoidei are of poly-phyletic origin. He bases this upon the study of the disposition of tbe gill-slits, of the translocation of the pectoral fin in relation thereto, among living forms, and of certain facts of palaeontology. I am, on the whole, disposed to accept the spirit of his conclusions ; but m y own researches suggest that, setting aside the Ceratopterina (which may possibly be related to the Rhinidce) and Pristis (which 1 Abhandl. d. Senckenbergsche Gesellsch. nat. Frankfurt, Bd. xv. 1888, 2 This vol.. p. 671. 3 P. Z. S. 1876, cf. pp. 52, 58, 59. * Sitzuugsb. d. Gesellsch. naturf. Freunde, Jhrg. 1890 (no. 3), p. 47. |
