12 bulletin: museum of comparative zoology. 



Klein ('68) called the outer edge of the " Brlicke " prefrontal, and the 

 inner and huider part of the same, principal frontal, 



Traquair (-65, pp. 27G, 277) summarizes the changes from the condi- 

 tion of the symmetrical type of skull as follows : 



" (1) The mesial vertical plane of the cranium has become inclined over to 

 the now binocular side, very slightly in the posterior part of the cranium, very 

 much in the region of the eyes (so that the original vertical interorbital septum 

 becomes now nearly horizontal), returning in the nasal region nearly to its 

 original vertical position in the turbot, but never doing so in the halibut or 

 plaice. 



" (2) In consequence of this, the middle line of the base of the skull remains 

 still comparatively straight; while the middle line of the upper surface, diverg- 

 ing from the apparent or pseudomesial line, curves round between the eyes, . . . 

 and returns to the middle in front. Having got in front of the eyes and nasal 

 fossae in the turbot, it again coincides, or nearly so, with the apparent middle 

 line ; but in the halibut, and still more in the plaice, the apparent and mor- 

 phological middle lines, if produced, would cross each other. 



" (3) In the anterior part of the cranium, the parts on the eyeless side of the 

 middle line of the base are, in all the Pleuronectidas, more developed than on 

 the ocular side. . . . 



" (4) On the top of the head the interocular parts of the frontal and pre- 

 frontal bones are more developed on the ocular side. The interocular process 

 of the frontal of the ocular side is always much stouter than that of the other 

 [eyeless side] bone, and always articulates with a corresponding process sent 

 back from the prefrontal. But the prefrontal of the eyeless side sends back 

 no process to articulate with the frontal of the same side, whose interocular 

 part, if examined in a series of flatfishes, gets smaller and smaller, till in the 

 plaice it seems almost gone. The same condition affects the morphologically 

 mesial plate of cartilage fonning the anterior part of the interocular st-ptum, 

 •which cartilage we have already seen to be chiefly developed on the ocular 

 side. 



" (5) To accommodate the two eyes, now both on one side of the head, the an- 

 terior parts of the frontal bones remain as a narrow bar, never widening out into 

 a broad arch as in the cod and other fishes. Accordingly, to maintain the 

 requisite stability of the cranium, a new bar or bridge of bone is formed (pseudo- 

 mesial) by the union of a process sent forwards from the anterior external 

 angle of the frontal of the eyeless side with one sent back from the correspond- 

 ing prefrontal. By means of this bar the uppei; eye becomes closed round by 

 a bony orbit, whose boundaries in the turbot consist of the interocular process 

 of the frontal of the eyeless side, the external angular process of the same bone, 

 the external angular process of the corresponding prefrontal, and a small por- 

 tion of cartilage in front. In the halibut and plaice, however, the nasal bone 

 comes to take part in the boundary of the orbit principally by a development 

 from its eyeless side; and in the latter fish, owing to the atruuhy of the inter- 



