THE CHORUS OF MAN’S STAGE 
especially because in its strata the complete records have 
not been preserved, we have little evidence of the kinds of 
creatures which then developed. 
It is in the rocks of the second great life period, that of 
the early Paleozoic time, divided in geological nomencla- 
ture into the ‘““Cambrian”’ and “Ordovician,” that numer- 
ous fossil specimens first become available, abundant, and 
beautifully preserved. These were the ages solely of the 
invertebrates, among them many creatures so different 
from present forms that they can not be said to have de- 
scendant representatives in the modern world. There 
was, for instance, the great family of the trilobites, which 
resembled superficially the lobster and the crab. The 
trilobites long ago became totally extinct, but they domi- 
nated the earlier part of the periods of which we are now 
speaking. Some of them, indeed, grew to large size, ex- 
ceeding eighteen inches in length. The name comes from 
the shape of the body, which presents a right, a left, and a 
middle prominent portion. Very beautiful in outline, 
with numerous delicate side organs almost fernlike in their 
detail, and provided with eyes and other sense organs, the 
trilobites seem to have been as complexly organized as 
many of the foremost of the invertebrates of the present 
day. They are not by any means primitive creatures. 
If we accept the theory of the gradual evolution of life 
forms, they must have had a very long ancestry. 
The trilobites counted among their contemporaries 
many kinds of shell-protected creatures, including several 
whose shells resemble greatly some of those of today. 
Thus we may regard the family of the present-day oyster 
and those of certain other bivalve mollusks as extremely 
ancient, though the species have changed from the ancient 
forms. 
Toward the close of the Ordovician period, the class of 
the cephalopods, now represented by the chambered 
nautilus, the octopus, squid, and their like, usurped the 
preeminence so long held by the trilobites, and continued 
[13] 
