444 MB. B. w. li. HOLT — STUDIES IN [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 Phgusia or, at any rate, in the Pleuronectid larvae which 

 Steenstrup and, subsequently, Agassiz, whether correctly or 

 incorrectly, attributed to that genus \ This process, however, 

 has been frustrated by the interposition of impenetrable bony 

 structures. The Sole has been sho\^1l, independently by Eaffaele^ 

 and Cunningham ^, to be one of the flat-fishes in which the migra- 

 tion of the eye is throughout external^, and this interposition of 

 the pseudomesial process coupled witli 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 \ 



Conclusiov. 



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 eiforts 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. Art8& Sci. xiv. 187S, p. 7. 



2 Mitth. zool. Stat. Neap. Bd. viii. tav. iii. figs. 8, 9. 



^ Journ. M. B. Assoc. N. S. vol. ii. no. iv. p. 327, pi. xiv. fig. 2. 



^ 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 Common Sole is probably one of the forms in vrhich the 

 dorsal fln, 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 

 {he. cit. fig. 103), after Malm, of a young Sole 12 nun. 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 .v-i/ is said to be " that part of 

 the future base of the dorsal fin imder 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 Rafiaele and Cunningham already referred to, Smitt is evidently 

 imacquainted 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 int^rneural 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 .r), is of some value, since it completely proves the 

 fallacy of the tlieory it is intended to support. I need hardly say that this 

 criticism is intended to apply only to the compiler, since Malm's observations 

 wei-e prior to those of Raffaele and Cunningham. 



^ The correctness of this view is confirmed by the observations of PfefTer, 

 aUuded to in the second part of this paper. 



