of the system itself, the species are changed; and these are
phenomena observed throughout all the subsequent systems or
geological eras; apparently arguing that, during the deposition of
all the rocks, a gradual change of physical conditions was constantly
going on. A varying temperature, or even a varying depth of sea,
would at present be attended with similar changes in marine life; and
by analogy we are entitled to assume that such variations in the
ancient seas might be amongst the causes of that constant change of
genera and species in the inhabitants of those seas to which the
organic contents of the rocks bear witness.

Some of the fossils of this system,--the cephalaspis, coccosteus,
pterichthys, holoptychius--are, in form and structure, entirely
different from any fishes now existing, only the sturgeon family
having any trace of affinity to them in any respect. They seem to
form a sort of connecting link between the crustacea and true fishes.

The cephalaspis may be considered as making the smallest advance from
the crustacean character; it very much resembles in form the asaphus
of lower formations, having a longish tail-like body inserted within
the cusp of a large crescent-shaped head, somewhat like a saddler's
cutting-knife. The body is covered with strong plates of bone,
enamelled, and the head was protected on the upper side with one
large plate, as with a buckler--hence the name, implying buckler-
head. A range of small fins conveys the idea of its having been as
weak in motion as it is strong in structure. The coccosteus may be
said to mark the next advance to fish creation. The outline of its
body is of the form of a short thick coffin, rounded, covered with
strong bony plates, and terminating in a long tail, which seems to
have been the sole organ of motion. It is very remarkable, that,