DR. TRAQUAIR ON THE ASYMMETRY OF THE PLEURONECTIDÆ. 267 
One of the simplest crania to be met with among the Pleuronectidæ is that of the 
Turbot (Rhombus maximus), of which illustrations are given in Plate XXIX. figs. 
1-7). This cranium, truncated behind and somewhat pointed in front, presents 
superiorly a longitudinal ridge (fig. 1, 8) which, though commencing posteriorly in the 
middle line, does not divide the head into two equal parts as it advances. On the con- 
trary, the anterior part of the cranium is broader to the left than to the right of this 
ridge, or of its supposed continuation forwards in a straight line; and this happens both 
because the skull anteriorly is actually considerably broader on the left than on the right 
side, and because the ridge itself deviates a little, though very slightly, towards the right 
side. This ridge supports the cephalic continuation of the dorsal fin. Posteriorly we at 
once recognize the brain-cavity, with its foramen magnum for the exit of the spinal 
cord, and various other foramina for cranial nerves. In front of the brain-cavity, and to 
the left side, is an oval orbit (5), which lodges the upper eye, the lower eye lying free 
beneath the lower bony margin of that orbit. In front of the orbit we recognize the 
nasal part of the cranium, with its two olfactory foramina, one on each side of the central 
cartilage (A). 
On comparing this cranium with that of the Cod, we observe that while in the latter 
the anterior and posterior parts of the skull are connected by two bars of bone—an 
inferior narrow one (basipresphenoid) and a superior flattened arch-shaped one (frontal)—- 
we have here three bars, the two upper bounding between them the orbit for the upper 
eye. To the left of these two bars, which forms the lower boundary of the orbit and lies 
between the two eyes, I shall give the name interocular; and to the other one, which 
bounds the orbit on the right side, and proceeds forward in the apparent middle line, I 
give the name pseudo-mesial. : 
In the Turbot and its congeners the eyes lie both on the left side usually—the upper 
one in the orbit, the lower free beneath the lower margin of that orbit, formed by 
the interocular bar. As, however, in some other groups of flatfishes the eyes are usually 
on the right side, to prevent confusion I shall, in the description of the bones of the 
flatfish-head, abandon the terms “ right" and * left" altogether, and use instead the 
terms “eyed” and “ eyeless.” 
Proceeding now to disarticulate the Turbot’s cranium, we find that posteriorly the 
bones are very little altered in their symmetry. 
Basioceipital (1). The long axis of this bone is somewhat obliquely placed as regards 
he transverse plane of the disk on its posterior surface for articulating with the first 
Yertebra, pointing a little towards the eyed side. 
Eroccipitals (2). Very symmetrical. 
ens oooipitals (4). The posterior projecting process is often longer on the eyeless 
ide, 
Alisphenoids (6). Very symmetrical. 
Mastoids (8), Very nearly equal in size and conformation. That of the eyeless side, 
however, is generally a little longer than that of the eyed. Ya 
Petrosals (15). These bones are much smaller than in the Cod, and lie quite super- 
lally. That of the eyeless side is in the Turbot always larger than the opposite one, 
