364 Prof. W. Thomson on Steenstrup's views 



surrounding bones of the head remain the same in all forms : in 

 all the orbit is excavated in the middle of the forehead, so that 

 the frontal and prefrontal bones of the two sides enclose the eye 

 between them, either immediately between them (as in most 

 Flounders) or so that a narrow plate of the frontal of the blind 

 side helps that of the eye side to circumscribe it beneath (as in 

 the Turbots, Soles, &c). 



A careful analysis of the head in the various groups of Floun- 

 ders shows that a considerable displacement of associated parts 

 has taken place in a definite direction, the displaced parts still 

 maintaining their original relations in position to one another; 

 the position of the upper eye in relation to its frontal bones (the 

 bones of the blind side) is, however, quite an abnormal one, and 

 cannot be explained by such a displacement or torsion in asso- 

 ciation with the surrounding parts. Under the generally ad- 

 mitted and correct supposition that the Flounders are not only 

 originally symmetrical, but that they retain this character, and 

 have an eye on each side of the head, for some time after exclu- 

 sion from the egg, " it becomes necessary to admit that the eye 

 of the blind side, from its original position at that side of the 

 head, has undertaken a movement, deeper and deeper in, under 

 the half-roof which the frontal bone of this side formed over it, 

 and has been brought up through its vault ; so that, in order to 

 find room for itself, it has partly separated the frontal bones 

 from one another, partly made its way through the mass of the 

 frontal bone itself." In other words, the eye could not possibly 

 have reached its final position without having passed obliquely 

 in, and up through the head, and come out at the other side. As 

 the nerves and muscles of the eye directly connect it with the 

 bottom of the orbit, the eye must have first passed under the 

 frontal bones, and then up through them. Were we to attempt 

 to explain the new position of the eye by torsion only, the nerves 

 and muscles must have passed over the frontal bones of the 

 blind side, and must lie permanently in that position, which is 

 not the case. The eye, in leaving its original site, however, has 

 attempted to carry the frontal bone of its own side along with 

 it ; but the greater part of this bone-mass has resisted, and has 

 remained in its place. The Flounder has thus acquired a firm 

 bony bridge extending from the snout to the back of the head 

 and the vertebral column, and of the utmost importance in con- 

 nexion with its new condition of equipoise. 



III. 



The passage of the eye obliquely up through the head is 

 finally proved by direct observations on young Flounders. Mi- 



