THE MUD-FISH 83 
fossil remains of small ganoids are found in Jurassic 
strata. Modern scaled fish, like the roach, dace, 
carp, and perch, which you will have seen in fresh 
water, and others which live in the sea, are of fairly 
recent origin. You do not find them in rocks older 
than the Cretaceous. Sharks and rays, however, 
have a long pedigree ; remains of their kind are 
found in Silurian deposits. These fish, as you 
know, have soft skeletons, and no bony plates to 
protect them, so there is no wonder that the only 
remains we generally find of them consist of spines 
Fie. 31.—TEetru or Extinct SHARKS FROM EocENE STRATA. 
and teeth. Even in the most ancient times the 
shark’s chief defence must have been its fierce and 
agoressive nature. 
I wonder if you have heard of the famous Aus- 
tralian Mud-fish named Ceratodus? It is found in 
the rivers of Queensland, and specimens may be 
seen in the London Zoological Gardens. Ceratodus 
is a genuine fish, and something more. Like 
ordinary fish it can breathe in water by means of 
gills, but unlike them it has an air-bladder which 
has become a lung. When the rivers are full of 
water it lives like other fish, but when the hot season 
arrives, and the rivers dry up, it makes a home in 
the mud, and breathes by its lung. A relation, or 
rather an ancestor, of this remarkable fish lived in 
