PISCES. 
-which does not exist at all upon the opposite 
side of the head, while anteriorly the anterior 
frontals (2) and the nasal bone (20) complete 
this part of the face. An orbital cavity is thus 
constructed upon the right or upper side of the 
head of the Pleuronectide, which suffices for the 
Jodgment of the two eyes, which thus take the only 
position in which both could be made useful.* 
Another equally remarkable arrangement is 
‘observable in the construction of the jaws of 
the Pleuronectide, which are in many genera 
very unequally developed on the two sides of 
the median line, only in this case the prepon- 
derance of developement is just the reverse of 
what exists in the orbital portion of the face, 
for here the bones of the right or upper side are 
small, while those of the left or inferior half 
are of considerably greater size and strength. 
Moreover, the former are but sparingly fur- 
nished with teeth, while the latter support the 
chief part.of the dental apparatus; so that by 
this structure the mouth becomes twisted toward 
the ground, and the teeth so disposed as to 
work most effectually in that direction. 
In the Syngnathide, Ostracions, and other 
Fishes, where the exoskeleton is inordinately 
developed, so as to-form a suit of bony armour 
in which the exterior of the body is completely 
eovered, the endoskeleton is proportionately 
weak and imperfectly formed, many of the 
Fig. 494. 
bones remaining in a rudimentary condition. 
This is well seen in the osteology of the Fly- 
ing Hippocamp, (Pegasus draco, fig. 494,) 
where the bones both of the head and trunk 
Seem to perform quite a secondary part as 
contrasted with the dense tegumentary frame- 
work covering the body. The whole face seems. 
* The above description of the structure of the 
ce in the Pleuronectide is derived from a disarti- 
culated skull of the Halibut, contained in the Mu- 
um of the Royal College of Surgeons in London. 
VOL. III. 
961 
to be composed of a kind of snout (a), derived 
from the external armour of the head, in which 
the orbits (6) are excavated, the intermaxillary 
bones (d) and the lower jaw (e) alone being 
recognisable. The gill covers (h) belong to the 
exoskeleton, while the hyoid apparatus (f) and 
branchial arches (g) are but very imperfectly 
formed. The osseous zone that sustains the 
pectoral fins consists of a single bone, which is 
so consolidated with the tegumentary skeleton 
as completely to separate the abdominal cavity 
from the branchial chambers; and in the piece 
m, which forms the basis of the fin itself, the 
usnal divisions are quite undiscernible. The 
vertebrae, both of the back and tail, (0, g,) are 
reduced to mere bony rings, while the pelvic 
circle, (p,) that nd gree the abdominal fins, (/,) 
is, like the rest of the skeleton, firmly con- 
nected with the external bony armour. 
In the construction of the anterior extremities 
a few peculiarities may be specified. 
Certain genera, more especially the Salmo- 
nide and the Cyprinide, have attached to the ra- 
dius and ulna upon their inner side a third bone, 
which by its anterior extremity is connected to 
the anterior margin of the os humeri, thus 
forming a kind of buttress to support the fin. 
In the Siluride there likewise exist three bones 
in the fore-arm which at an early period be- 
come consolidated to each other, probably on 
account of the great strength requisite 
in that race of Fishes to support the great 
spinous ray of the pectoral fin. In the 
urenide, where there are but two 
bones, these are suspended to the arch of 
the shoulder at the point of junction be- 
tween the scapula and the humeral bone. 
In species that have no pectoral fins the 
radius and ulna do not exist. 
There still remains to be noticed a 
long styliform bone generally composed 
of two pieces (fig. 437, 49, 50, p. 827, 
and fig.493, 10), of which the upper piece 
(49), more or less flattened in shape, is 
suspended from the os humeri (48), to 
the posterior and superior part of the inner 
surface of which it is adherent. This 
styliform bone runs backwards along the 
side of the body behind the pectoral fin, 
and is plunged to a greater or less extent 
amongst the flesh.* Some anatomists 
have regarded this process as the homo- 
logue of the clavicle, but from the posi- 
tion which it occupies, running backwards, 
it seems rather to represent the coracoid 
bone, which is in this case lost among the 
muscles on account of the want of a 
sternum to which it might be articulated. 
It sometimes happens that this bone unites with 
that of the opposite side, and occasionally 
is of such length and strength as to reach 
backwards as far as the commencement of the 
anal fin. A not less curious disposition of this 
bone is observed in Batrachus, where the — 
rior division extends upwards beyond the hu- 
merus to be connected with the spinous process 
of the first vertebra. 
* Cuv. Hist. des Poissons. 
3. 
