52 
THE SILURIAN BEACH. 
some species, of which others are entirely desti- 
tute, — and the smooth outline of some, while in 
others the surface is broken by a variety of ex- 
ternal ornamentation. Such are a few of the 
more prominent differences among them. But 
the general structural features are the same in 
all. The middle region of the body is always 
divided in uniform rings, lobed in the middle so 
as to make a ridge along the back with a slight 
depression on either side of it. It is from this 
three-lobed division that they receive their name. 
The subjoined wood-cut represents a character- 
istic Silurian Trilobite. 
There is no group more 
prominent in the earliest cre- 
ations than this one of the 
Trilobites, and so exclusively 
do they belong to them, that, 
as we shall see, in proportion 
as the later representatives 
of the class come in, these 
old-world Crustaceans drop 
out of the ranks, fall behind, 
as it were, in the long pro- 
cession of animals, and are left in the ancient 
deposits. Even in the Carboniferous period but 
few are to be found : they had their day in the 
Silurian and Devonian ages. In consequence of 
their solid exterior, the preservation of these ani 
