266 DR. TRAQUAIR ON THE ASYMMETRY OF THE PLEURONECTIDE. 
themselves. It will also be necessary to compare the cranium and its parts with that 
of a symmetrical fish ; and, for this purpose, we may select the cranium of the common 
Cod (Gadus morrhua), a fish belonging to the same suborder (Anacanthina) of the Telostei 
asthe Pleuronectidæ. As a standard for such comparison this cranium will do very well, 
the differences between its plan of structure and that of the flatfish-skull being really im- 
material. 
In such a cranium (Plate XXIX. fig. 16) we find three principal parts, each connected 
with one great organ of special sense. 
l. À posterior cavity (z) containing the brain and organ of hearing, this cavity being, 
in the macerated skull, widely open in front, and showing a “foramen magnum" be- 
hind for the exit of the spinal cord. Into its composition enter the basilar, exoccipital, 
paroccipital, supraoccipital, petrous, mastoid, orbitosphenoid, alisphenoid, part of the 
basipresphenoid, and part of the mid or great frontal bones, also a considerable 
quantity of unossified cartilage. 
2. A middle or interorbital part (v), consisting of part of the frontal bone above, part 
of the basipresphenoid below, with a fibrous membrane (the “ septum interorbitale ") ex- 
tending vertically between them. This septum is formed by the coalescence of two 
fibrous laminze, which close to a considerable extent the anterior opening of the brain- 
case, and also complete a groove on the under surface of the frontal bone into a canal 
which continues the brain-cavity as far forwards as the nose, and lodges the crura of the 
olfaetory bulbs. Note, that while the basipresphenoid below remains a narrow bar, the 
frontal bone above forms a large broad arched plate, which both contributes to the general 
stability of the cranium and forms very efficient roofs for the orbits. 
3. An anterior or nasal part (X), which contains no cavity, but presents two openings 
for the olfactory nerves—one on each side of a central mass of cartilage. This part of the 
cranium consists of four bones—the vomer below, the nasal bone above, and the two pre- 
frontals on each side, each of which is notched internally for an olfactory nerve. All 
these bones are supported by the central mass of cartilage already referred to. 
Another well-known circumstance in the structure of the skull of the Cod, as of other 
fishes, must be noticed, viz. that, when the individual bones are disarticulated, certain of 
them can be removed without in the least interfering with the primordial cartilage, of which 
a considerable quantity still remains. In the Cod these superficial bones, or “ Deck- 
knochen,” are invariably the vomer, the basipresphenoid, the frontal, the parietals, and 
the petrosals. The other bones are so intimately connected to the cartilage, that they 
cannot be separated without tearing it and carrying away pieces of it in their substance. 
They are the basioccipital, paroccipitals, exoccipitals, supraoccipital, mastoids, post- 
. frontals, alisphenoids, orbitosphenoids, prefrontals, and nasal. 
i The only differences worthy of note between the general plan of the Cod’s skull as 
given above and that in the flatfishes are, that the two halves of the single frontal bone 
x the Cod = represented in the latter by two distinct bones, and that in the flatfishes 
he membranous interorbital septum contains no tubular prolongation of the nw 
pi à In the Pleuronectidæ and in the Gadidæ the relations of the bones to the 
cartilage are identical. 
