82 THE OPEN BOOK OF NATURE 
Pterichthys belonged to the earliest known type of 
fishes, the Ganoids (Greek, ganos = brightness, ezdos 
=appearance), which were exceedingly numerous in 
early geological times, but have few living repre- 
sentatives. The sturgeon is an existing species, and 
so also is the garpike, which is found in the American 
lakes. Modern ganoids live in fresh-water lakes and 
rivers, and it is conjectured that the ancient ganoids 
were fresh-water fish, but many of them must have 
visited the seas just as our salmon do. The ganoids 
were like armoured cruisers, being clothed with 
plates of bone and enamelled scales, by which they 
(From Oxford Clay.) 
were fortified against the attacks of enemies. We 
build armour-plated battleships at great cost of 
money and labour ; Nature furnished those early fish 
with defensive armour ‘“‘ without money and without 
price.” The fact that they were so well defended 
indicates their need of defence; there must have 
been an abundance of ferocious creatures eager to 
prey upon them. The struggle for existence has 
been keen in all ages. The ganoid fishes evidently 
flourished in Paleozoic and Mesozoic times, but 
after the latter Era they seem to have waned. They 
were numerous in the Carboniferous Period, and 
