GRAPTOLITES—TRILOBITES. 69 
fusion of life met with in the Silurian and Devonian 
strata presupposes an immeasurably long antecedent 
period during which life had already existed and gradu- 
ally increased to the multitudes of the Silurian era. 
We discover in it but scanty remains of marine plants, 
and only marine animals; but these are so heterogeneous 
and varied in form, that they alone would oblige us to 
infer the existence of coasts, shallow or deep oceanic 
regions, and a number of geographical conditions on 
which we see the variety and extent of animal life to be 
dependent, Besides numerous forms of 
corals more nearly allied to still exist- 
ing families, we find the quite peculiar 
group of Graptolites (fig. 9), which, 
although not actual polypes, might be 
ranged next to the so-called Medusa- 
polypes, and thus justify the inference 
that preparation was being made for the 
appearance of the higher forms of the 
Ccelenterata, the Meduse. 
The Articulata are represented by the 
Trilobites (fig. 10, Trilobites remipes), 
a crab-like form which recalls the pre- 
sent group of the Lamellibranchiata, but Fic. 9. 
has not hitherto admitted of any closer definition, as 
in none of the many thousand specimens examined, 
of the forms (about 2000) known in the Silurian and 
Devonian strata, have the legs been preserved. In these 
three-lobed crabs, the head, trunk, and tail distinctly ap- 
pear, as well as the threefold transverse division. The 
two composite eyes already indicate a high grade of 
organization. The power of rolling themselves up, 
