DR. TRAQUAIR ON THE ASYMMETRY OF THE PLEURONECTIDE. 281 
on the ocular side, so that the direction in which the lower jaw works is towards the 
eyeless side. (See Plate XXX. fig. 6.) 
4. The conformation of the jaws themselves is also very important in connexion with 
the obliquity of the mouth. The superior maxillary bone of the eyed side is to some 
extent smaller than that of the eyeless side. But the intermaxillary of the eyed side is 
very mueh smaller than its fellow; its ascending process is at a more obtuse angle 
to the body of the bone, which bears only 4—7 teeth, while the bone of the eyeless side, 
stout and strong, with its ascending process, set at nearly a right angle to its body, is set 
with 25-30 teeth or more. The lower jaw of the ocular side, rather flat, is shorter than 
that of the eyeless side; its dentary bone bears, like the corresponding intermaxillary, 
only 4-7 teeth. "The longer lower jaw of the eyeless side has, on the other hand, its 
dentary part much curved and set with 25-35 teeth; so that not only does the mouth, 
when opened, point to the eyeless side, but that side of the mouth is more arched and 
prominent, even when shut, and contains almost all the teeth. 
In Plate XXX. fig. 4, is figured the palato-suspensory and opercular apparatus of the 
oeular side of the Plaice, seen laterally ; and in fig. 5 that of the eyeless side. Fig. 7 re- 
presents the cranium and palato-suspensory apparatus of both sides, seen exactly from 
the front; while fig. 6 gives a view of the lower jaw and opereular apparatus of both 
sides, seen from below. 
HI. On the Superficial Face-bones and on the Distribution of the Slime-canals. 
We have still to consider whether there be in the Pleuronectidz any representatives 
of the supratemporal and suborbital ranges of bones, and of those bones called by Cuvier 
“nasal,” by Owen * turbinal.” 
In osseous fishes generally these bones are intimately connected with a system of 
dermal tubular organs, the “ mueus-" or * slime"-canals ; and hence it will be necessary 
for us also to study the relations and arrangement of these canals in the Pleuronectidæ. 
e arrangement of these canals on the heads of osseous fishes follows, on the whole, a 
very definite plan* ; and if we adhere to the Cod as our standard of comparison, we shall 
find how completely the plan of the arrangement of the mucus-canals in the Pleuronec- 
tidæ corresponds with that of the same organs in the Cod, and how that plan has been 
modified entirely in accordance with the theory of the Pleuronect eranium already given. 
The Symmetrical Arrangement in the Cod.—The plan of this arrangement I have 
represented in a diagram (Plate XXXII. fig. 1). The mucus-canal of the lateral line (a a), 
Supported all the way along by peculiarly modified scales, extends on to the head, runs 
ong grooves in the mastoid, postfrontal, and frontal bones, and then, lodged in the 
Srooved turbina] (w), terminates near the end of the snout, and to the inner side of the 
nostril. On the surface of the frontal bone it forms a commissure (e) with its fellow of 
the opposite side. On its way, it gives off the following branches :— 
* The arrange ^ ; ibed by Monro in his work, ‘The Struc- 
ture and M Hoan pug d prp id A ue Animals,‘ Edinburgh, 1785 ; 
Les also by Stannius in a paper, “Ueber die Knochen’ des Seitenkanals der Fische,” Froriep’s Neue Notizen, Bd. 
zum. S. 97-100 (April 1842), 2R9 
