-the
presence or absence of prongs on the shield and of spines along the
sides of the body,--appendages on the head in some species, of which
others are entirely destitute,--and the smooth outline of some, while
in others the surface is broken by a variety of external 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 characteristic Silurian
Trilobite.

[Illustration]

There is no group more prominent in the earliest creations 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 procession 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 animals
is very complete; and their attitudes are often so natural, and the
condition of all their parts so perfect, that one would say they had
died yesterday rather than countless centuries ago.

Their geological history has been very thoroughly studied; not only are
we familiar with all their adult characters, but even their embryology
is well known to naturalists. It is, indeed, wonderful that the mode of
growth of animals which died out in the Carboniferous period should
be better known to us than that of many living types. But it is
nevertheless true that their embryonic forms have been found perfectly
preserved in the rocks, and Barrande, in his "Systeme Silurien de la
Boheme," gives us all the stages of their development, from the time
when the animal is merely sketched out as a simple furrow in the embryo
to its mature condition. So complete is the sequence, that the plate on
which their embryonic changes are illustrated contains more than thirty
figures, all representing different phases of their growth. There is not
a living Crab represented so fully in any of our scientific works as is
that one species of Trilobite whose whole story Barrande has traced