On the Development of the Pleuro7iectidce, 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-6sh 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 
" 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 der Vergl. Anatomie. Thl. ii. Halle, 1824. 
t See tlie author's forthcoming paper " On the Asymmetry of the Pleuro- 
nectidfe" in the Transactions of the Linnean Society, 1865. 
X System der Vergl. Anatomie. Thl. ii. Halle, 1824. 
