A. BISULCATUS.

(_Lias._)]

[Illustration: Fig. 82.

BELEMNITELLA MUCRONATA.

(_Chalk._)]

[Illustration: Fig. 83.

BELEMNITES SULCATUS.

(_Oolite._)]

In the limits to which I have restricted myself, I have been able to do
little more than simply to chronicle the successive eras in which the
various classes and divisions of the organic kingdom, vegetable and
animal, make their appearance in creation. I have produced merely a
brief record of the various births, in their order, of that great family
whose father is God. And in pursuing such a plan, much, of necessity,
must have been omitted. I ought perhaps to have told you, that very
rarely, if ever, do the master forms of a period constitute the
prevailing or typical organisms of its deposits. Of the three great
divisions of which the geologic scale consists,--Palaeozoic, Secondary,
and Tertiary,--the first, or ichthyic period, is marked chiefly, not by
its great fishes, but by the peculiar character of its brachipodous and
cephalopodous mollusca, and in its earlier stages by its three-lobed
crustaceae; the second or reptilian period was emphatically the period
of the ammonite and belemnite; while the third and last, or mammalian
period, was that of gastropodous and conchiferous molluscs, impressed,
generically at least, by all the features of the group which still
exists in our seas. Save in a few local deposits, fishes do not form the
prevailing organisms in the formations of the age of fishes; nor
reptiles in the formations of the age of reptiles; nor yet mammals in
the formations of the age of mammals. Nay, it is not improbable that the
recent or human period may be marked most prominently in the future,
when it comes to exist simply as a geologic system, by a still humbler
organism than most of these molluscs. On almost all rocky shores a line
of pale gray may be seen at low water, running for mile after mile along
the belt that has been laid bare at the bases of the cliffs by the fall
of the tide. It owes its pale color to millions of millions of a small
balanus (_B. balanoides_), produced in such amazing abundance in the
littoral zone as to cover with a rough crust every minute portion of
rock and every sedentary shell. Other species of the same genus (_B.
crenatus_ and _B. porcatus_) occupy the depths of the sea beyond; and
their remains, washed ashore by the waves, and mingled with those of the
littoral species, form often great accumulations of