PEIM^VAL FISH. 207 



begin tlie 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. Primaeval fish, Ganoid fish, and Osseous fish. 

 The oldest of these, where the original form has been most 

 faithfuUy preserved, is that of the Primceval fish (Selachii). 

 Of these there still exist Sharks (Squali), and Rays 

 (Ilajse), which are classed together as cross-mouthed fishes 

 (Plagiostomi), and the strange and grotesquely formed Sea- 

 cats, or Chimceracei (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 palseohthic period. 

 Unfortunately all Primaeval fish possess a cartilaginous, 

 never a completely osseous skeleton, which is but little, if 

 at aU, capable of being petrified. The only hard parts of 

 the body which could be preserved in a fossil state, are the 



