ARCHEAN AND PALEOZOIC LIFE 411 
forms of marine life, which were so abundant in the 
Paleozoic seas, have continued to decrease since then, 
until at the present time they are rare in the ocean, 
and in their perfect development are mostly confined 
to the deep sea. 
Another type of ocean invertebrate, which had 
some importance in the preceding ages, and became 
especially prominent in the Silurian, is that of the: 
cephalopods (Figs. 235 (2 and 5) and 258). In the 
Paleozoic time these creatures were mainly represented 
by forms which dwelt in long and straight shells, which 
they constructed. Later, during the Mesozoic time, the 
cephalopods were dwellers in coiled shells, or else were 
creatures without a shell (Figs. 247 and 248). 
These creatures still live in the modern ocean, where they 
are represented by the squid, cuttle-fish, and devil-fish, which are 
types that have lost the shell, and also by the nautilus and argo- 
naut, which are forms that have retained the coiled shell. The 
development of variations in the shells and skeletons of these 
animals, has been most interesting. Indeed, it forms one of the 
best proofs of evolution which is furnished by the fossil record of 
animal life. 
From the Silurian rocks is found the first definite 
record of the existence of true fishes upon the earth. 
These are found as fossils in considerable numbers 
among the strata of this age; and they present such 
variety and high development, that it seems difficult 
to believe that they really first appeared upon the earth 
