412 ELEMENTARY GEOLOGY 
in this complete form. According to the theory of evo- 
lution, which receives so much support from the life 
record in the rocks, there should have been ancestors 
of less complex structure. Possibly the progenitors 
of these fishes had no true bones or scales which 
could be preserved in the rocks, and hence have left 
us no record of 
their existence. 
The types of Pal- 
eozoie fishes are 
quite different from 
those of the present 
time, although some 
shark-like forms 
then lived. Most 
of the Silurian fishes 
were covered with 
Fic. 236. 
an armor of plate- 
A crustacean from the Silurian beds. (Euryp- 
terus lacustris.) like scales, so that 
they are graphically 
called mail-coated fishes (Fig. 259). Among the 
modern species, the sturgeon is the best-known rep- 
resentative of mail-clad fishes. 
Considerable development of life is noticed on the 
land, although none of the higher groups of animals 
or plants have yet appeared. The plants have not 
left us a very perfect record of their existence, and 
