10 PROCEEDINGS OF THE AMERICAN ACADEMY 



mens which furnished him the materials for his paper on Plagusia. 

 The present transparent species evidently belonged to this genus (Pla- 

 gusia) ; and I had thus succeeded in actually tracing, in one and the 

 same individual, the passage of the right eye to the left side through 

 the head. 



If we now compare this method of transfer of the eye through the 

 head with the transfer previously described round the frontal bone on 

 the exterior of the head, we can readily see that the difference is not 

 as great as it would appear at first sight. Were we to imagine this 

 species of Plagusia with a dorsal, stopping in the anterior median line 

 behind the posterior edge of the eyes, the transfer would then take 

 place exactly as in the case of the common Flounders. The right 

 eye would travel round the frontal, without having to sink into the 

 tissues ; and, if subsequently to the transfer of the right eye to the 

 binocular side, the anterior portion of the dorsal were to extend in 

 advance of the anterior edge of the eyes to the intermaxillary, we 

 should then obtain a result identical with that described before, and 

 one which actually occurs in precisely this manner as we have seen 

 in a number of Flounders ; and the mere resorption of the tissues 

 at the base of the anterior part of the dorsal, while interesting as a 

 short-cut to an end, is not of so great physiological value, or so im- 

 portant as a difference in the method of the transfer of the eye, as 

 appears on a first examination. 



Owing to the transparency of this Plagusia, several interesting struc- 

 tural details could readily be followed, which only tedious manipulation 

 would have demonstrated in the other more opaque species, of which 

 the development is here given. Among these were the great length of 

 the optic nerve, which allows, as it were, sufficient slack to be taken in 

 during the transfer of the eye from the right to the left side (PL X. figs. 

 4, 8, 9), so as apparently not to interfere in any way with the sight of 

 the right eye ; also, the immense accumulation of muscular bands 

 forming the sheath of the orbits of the eyes, and providing for the 

 great variety and range in the movements of the eyeball and lids (PI. 

 X. figs. 3, 4, 5, 8, 9) ; also the direct and very active circulation taking 

 place to and from the heart with the cavity of the orbit of the eyes. 

 (See PI. X. fig. 9, where the direction of the arrows shows the 

 course of this current.) The presence of this circulation of a so- 

 called ocular heart can be readily traced in the adult of our Halibut. 



The Flounders have thus far only been found in the most recent 

 geological deposits : they seem to belong peculiarly to the present 

 period. It is certainly remarkable that no Flounders should have 



