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Show 444 MR. E. W. L. HOLT-STUDIES I N [May 1, There is, as it were, an attempt on the part of the eye to get to the right side of the head by passing through the tissues, as in the larval Plagusia or, at any rate, in the Pleuronectid larvae which Steenstrup and, subsequently, Agassiz, whether correctly or incorrectly, attributed to that genus1. This process, however, has been frustrated by the interposition of impenetrable bony-structures. The Sole has been shown, independently by Eaffaele2 and Cunningham3, to be one of the flat-fishes in which the migration of the eye is throughout external1, and this interposition of the pseudomesial process coupled with the constant presence in all flat-fish of one of the accessory visual organs on the blind side of the head, lends support to the view that the union of the ectethmoid and sphenotic of the blind side cannot take place until after the eye has passed the ridge of the head °. Conclusion. It remains for us to consider, as briefly as may be, what light our specimen throws upon the theory of Pleuronectid evolution. The rotation of such part of the skull as is in intimate connection with the eyes is attributed by Mr. Cunningham (Treatise &c. pp. 52, 53) to the heredity of a character acquired by the efforts of an originally symmetrical fish to look with its lower eye beyond the edge of its head: in fact, " to the accumulation, by inheritance, 1 " Development of the Flounders," Proc. Am. Ac. Arts& Sci. xiv. 1878, p. 7- 2 Mitth. zool. Stat. Neap. Bd. viii. tav. iii. figs. 8, 9. 3 Journ. M . B. Assoc. N. S. vol. ii. no. iv. p. 327, pi. xiv. fig. 2. 4 It is somewhat surprising, until, by perusal of his work, one has become accustomed to his consistent neglect of recent authors other than Scandinavian, to find that Professor Smitt (Hist. Scand. Fishes, ed. ii. 1893, p. 365) still considers that the C o m m o n Sole is probably one of the forms in which the dorsal fin, or at all events its predecessor, the embryonic vertical fin, extends so far forward on the dorsal edge of the head that the eye must force its way under the base of this fin. In support of this assertion he appends a figure (loc. cit. fig. 103), after Malm, of a young Sole 12 m m . long, in which both eyes are already on the right side of the head. The base of the permanent dorsal fin is clearly enough shown to commence at a point marked x, which is about opposite the posterior margin of the upper eye ; in front of this a point y is shown on the dorsal profile of the head, and x-y is said to be " that part of the future base of tbe dorsal fin under which the left (upper) eye has probably passed." So far as one can judge by the drawing, all that part of the dorsal profile in front of the point x is in reality formed by the skin simply, and not by an anterior prolongation of the embryonic fin. Besides overlooking the work of Raffaele and Cunningham already referred to, Smitt is evidently unacquainted with those observations of the Italian zoologist which show that the cephalic prolongation of the dorsal fin is brought about by a forward migration of the fin-rays and interneural elements, and not by the formation of new fin-rays. The figure, showing as it does the anterior end of the true dorsal fin (at the point x), is of some value, since it completely proves the fallacy of the theory it is intended to support. I need hardly say that this criticism is intended to apply only to the compiler, since Malm's observations were prior to those of Raffaele and Cunningham. 1 The correctness of this view is confirmed by the observations of Pfeffer. alluded to in the second part of this paper. |