PISCES. 233 
peculiar species of Pegasus is P. draco (pl. 81, fig. 17), from the Indian 
seas. 
The order of fishes we have now to consider is one of the most 
interesting in the whole department of Ichthyology. It includes forms, 
which, predominating exclusively at the dawn of vertebrate creation, 
have long since passed away, leaving nothing but the casts or imper- 
fect skeleton remains to bear witness of animals which once inhabited 
the waters of a Paleozoic age. But few genera of any of these families 
remain to testify as to what was once the rule, and not, as now, the 
exception in nature. We may, with some authors, divide all fishes into 
Teleostei, Ganoidei, and Selachiit. The Teleostei are fishes with a bony 
skeleton, the bones of the head being also united by sutures. Other 
characters are to be found in the structure of the heart, where the bulbus 
arteriosus is simply an expansion of the aorta, and does not, as in the 
Ganoids and Selachians, pulsate independently. There are no traces of 
the transverse fasciculi of the heart, but only pale fasciculi of soft fibres, 
which taper gradually into an uniform layer of the artery. There are 
also two opposite valves, separating the bulbus from the heart. The 
Cyclostomes have these valves, but are without the swelling of the bulb. 
Deferring for the present the consideration of the comparative peculiarities 
of the Selachian division, we proceed to the Ganoidei, the second division 
of this classification, the ninth order of the one we have already adopted. 
They form a true bond of union between the Teleostei and the Selachii, 
having properties common to both. Their most conspicuous external 
characters are the possession of angular bony or horny scales, covered 
with enamel. Their internal peculiarities consist in the multiple valves 
and the muscular investment of the aorta, in the non-decussating optic 
nerves, in the free gills and operculum, and in the abdominal ventral fins. 
Other characters no less important, but more variable in their appearance, 
are to be found in the single or double series of spinous plates or imbrica- 
tions in the interior edge of the tail, and in the inequilobal or heterocercal 
tail, a structure in which the vertebral column, instead of running out to 
the middle of the caudal fin, has its termination in the upper lobe. This 
feature is highly characteristic of the Plagiostomes of the present day. 
Some of these, besides the pseudo-branchie, have an additional organ of 
respiration in an opercular gill. Finally, to the above mentioned charac- 
ters are to be added more or less of the following: the spiral valve in the 
intestine, the air-duct of the swimming bladder, the discharge of the ova 
from the abdominal cavity through tubes, and the partly imperfect 
skeleton; as also the tubular, angular, or round enamelled scales, or bony 
plates, where any covering whatever exists. 
Thus of cartilaginous fishes, they have the accessory gill before the first, 
the spiracles; the valves and muscles of the aorta, the vascular distribution 
of the pseudo-branchie, the oviducts, and the peculiarity of the optic nerves. ~ 
Of osseous, or bony fishes, they have these characters: the structure of the 
nose, the operculum, and the free gills. "There is a swimming bladder in 
all Ganoids, with a free air-duct, and without a rete mirabile. 
437 
