PRIMAVAL FISH. 207 
begin the series of Double-nostriled animals, is distinguished 
from the other six classes of the series by the swimming 
bladder never developing into lungs, but acting only as a 
hydrostatic apparatus. Agreeing with this, we find that 
in fish the nose is formed by two blind holes in front of 
the mouth, which never pierce the palate so as to open 
into the cavity of the mouth. In the other six classes of 
double-nostriled animals, both nostrils are changed into air 
passages which pierce the palate, and thus conduct air 
to the lungs. Genuine fish (after the exclusion of the 
Dipneusta) are accordingly the only double-nostriled 
animals which exclusively breathe through gills and never 
through lungs. In accordance with this, they all live in 
water, and both pairs of their legs have retained the original 
form of paddling fins. 
Genuine fish are divided into three distinct sub-classes, 
namely, Primeval fish, Ganoid fish, and Osseous fish. 
The oldest of these, where the original form has been most 
faithfully preserved, is that of the Primeval fish (Selachii), 
Of these there still exist Sharks (Squali), and Rays 
(Rajz), which are classed together as cross-mouthed fishes 
(Plagiostomi), and the strange and grotesquely formed Sea- 
cats, or Chimcracet (Holocephali). These primary fish of 
the present day, which are met with in all seas, are only 
poor remains of the prevailing animal groups, rich in forms, 
which the Selachii formed in the earlier periods of the 
earth’s history, and especially during the palzeolithic period. 
Unfortunately all Primzval fish possess a cartilaginous, 
never a completely osseous skeleton, which is but little, if 
at all, capable of being petrified. ‘The only hard parts of 
the body which could be preserved in a fossil state, are the 
