till his whole shell was divided into successive
chambers, all of which were connected by a siphon. Some of the shells of
this kind belonging to the Silurian deposits are enormous: giants of the
sea they must have been in those days. They have been found fifteen
feet long, and as large round as a man's body. One can imagine that the
Cuttle-Fish inhabiting such a shell must have been a formidable animal.
These straight-chambered shells of the Silurian and Devonian seas are
called Orthoceratites (see wood-cut below). We shall meet them again
hereafter, under another name and with a different form; for, as they
advance in the geological ages, they not only assume the curved outline
with ever closer whorls till it culminates in the compact coil of the
Ammonites of the middle periods, but the partitions, which are perfectly
plain walls in these earlier forms, become scalloped and involuted along
the edges in the later ones, making the most delicate and exquisite
tracery on the surface of the shell.

Of Articulates we find only two classes, Worms and Crustacea. Insects
there were none,--for, as we have seen, this early world was wholly
marine. There is little to be said of the Worms, for their soft bodies,
unprotected by any hard covering, could hardly be preserved; but,
like the marine Worms of our own times, they were in the habit of
constructing envelopes for themselves, built of sand, or sometimes from
a secretion of their own bodies, and these cases we find in the earliest
deposits, giving us assurance that the Worms were represented there.
I should add, however, that many impressions described as produced by
Worms are more likely to have been the tracks of Crustacea.

But by far the most characteristic class of Articulates in ancient times
were the Crustaceans. The Trilobites stand in the same relation to the
modern Crustacea as the Crinoids do to the modern Echinoderms. They were
then the sole representatives of the class, and the variety and richness
of the type are most extraordinary. They were of nearly equal breadth
for the whole length of the body, and rounded at the two ends, so as
to form an oval outline. To give any adequate idea of the number and
variety of species would fill a volume, but I may enumerate some of
the more striking differences: as, for instance, the greater or less
prominence of the anterior shield,--the preponderance of the posterior
end in some, while in others the two ends are nearly equal,-