On the Development of the Pleuronectidce. 217 



tween the long processes of the frontal bones, after the 

 manner of cyclopean malformations. The view which 

 occurred to him is then in accordance with the second theory 

 suggested in the beginning of this paper, but which a care- 

 ful examination of the osteology of a series of different 

 species of flat-fish will easily show to be untenable. 



It is, however, to Meckel* that we owe almost all our pre- 

 vious knowledge of the subject, as far as the structure of the 

 adult Pleuronect is concerned. He recognised correctly the 

 homologies of the various cranial bones, with those of the 

 symmetrical fish, and was undoubtedly the first who saw 

 clearly that, according to the first theory already suggested, 

 the two eyes of the flat-fish are brought round to one side, 

 by the turning or twisting round of the middle line of the 

 head ; but his notions as to the prolongation of the dorsal 

 fin along the head, and its strange position as regards the 

 eyes, are unsatisfactory, as we shall see afterwards.f 



Van Beneden,J in 1853, published a paper, the first in 

 which notice has been taken of the development of the 

 Pleuronectidse. In this paper, he has described a young 

 turbot, taken probably soon after its extrusion from the egg, 

 and in which that stage of development does not seem yet 

 to have been reached, when the eyes become both placed on 

 one side. " In this young fish the mouth is perfectly sym- 

 metrical, the eyes are still on the two sides of the head, but 

 the left is about to pass over to the right side, — the nostrils 

 are still symmetrical. The rays of the dorsal fin only yet 

 descend to the middle of the cranium, afterwards they 

 stretch on in front of the eyes, but it is necessary first, that 

 the twisting of the head should have taken place in the ver- 

 tebral column." 



To these observations, he adds the result of some made 

 f on a turbot of nearly adult size, in which the process of 

 torsion is arrested when the eye has arrived at the middle 

 line. The rays of the dorsal fin have not yet descended 



• System rter Vergl. Anatomie. Thl. ii. Hallo, 1824. 

 t Seo the author's forthcoming paper "On the Asymmetry of the Pleuro- 

 ■tetidflB" in the Transactions of the Linnean Society, 18G5. 

 I System der Vergl. Anatomie. Thl. ii. Halle, 1824. 



