PISCES TELEOSTEI 
(OSSEOUS FISHES). 
Skeleton complete and, usually, ossified. Olfactory organ double. The optic nerves 
only decussate, and do not form a chiasma. Only two valves" in the arterial bulb. 
In addition to their other characteristics, the com- 
paratively lax arterial bulb of the two-chambered heart, 
from which the blood passes to the arteries of the 
branchial arches, and the simple decussation of the 
optic nerves, which, at their exit from the brain, only 
cross each other without fusing together their fibres, 
place these fishes lower on the scale of development 
than the Chondrosteans, Chondropterygians and the 
Ganoids (the latter unrepresented in the Scandinavian 
Fauna), which orders come nearer the Batrachians. On 
the other hand, the double olfactory organ, which the 
osseous fishes possess in common with the above-men- 
tioned higher orders, separates them from the Cyclo- 
st-omes, which are represented in Scandinavia by the 
lampreys, while the skull places them as well as all 
other true fish, as vertebrates, high above the Cirro- 
stomes, the sub-class to which the lancelet belongs. 
Thus, while it is true that the osseous fishes do not 
occupy the highest rank among fish with respect to 
their general degree of development, regarded as fish 
they are nevertheless, above all the others, distinctly 
marked by their usually ossified skeleton and the ma- 
nifold development of their organs to meet the require- 
ments of aqueous life. 
TELEOSTEI PHYSOCLYSTI. 
Osseous fishes with the air-bladder, if present, closed (without pneumatic duct) at least in the adult. 
The air-bladder, which corresponds anatomically, 
though not physiologically, to the lungs of the higher 
vertebrate animals, is originally an outgrowth of the 
digestive canal, but loses in the most highly developed 
fishes its immediate connection with this canal. Such 
a change is in itself a sufficient indication of a more 
advanced degree of development, and has consequently 
full claim to be regarded in a systematic scheme of 
arrangement. In certain cases where the air-bladder 
is absent — as, for instance, in the common mackerel, 
whereas it exists in the so called Spanish mackerel, 
which is nevertheless a very closely allied species — 
we must be guided by other resemblances to assign the 
fish its place among the Physoclysts. 
a Some Clupeoids form an exception, as they retain a rudiment of the Ganoid conus arteriosus at the base of the bulb. Cf. Boas: 
Om conus arteriosus hos Butirinus og hos andre Teleostei. Vidensk. Meddel. Naturli. For. Kbnhvn 1879 — -80, p. 333. Generally too 
rudiments are to be found of two more valves in the arterial bulb between the others; but in Orthagoriscus viola (cf. Wellenbergh: Observ. Anat. 
de Orth, mola, Lugd. Batav. 1840) these rudiments are true valves. 
Scandinavian Fishes. 
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