sign which, amid endless
diversity, pervades all nature. The mollusca of the Silurians ranged
from the high cephalopoda, represented in our existing seas by the
nautili and the cuttle-fishes, to the low brachipods, some of whose
congeners may still be detected in the terebratula of our Highland lochs
and bays, and some in the lingulae of the southern hemisphere. The
cephalopods of the system are all of an obsolete type, that disappeared
myriads of ages ago,--a remark which, with the exceptions just
intimated, and perhaps one or two others, applies equally to its
brachipods; but of at least two of its intermediate families,--the
gasteropoda and lamellibranchiata,--several of the forms resemble those
of recent shells of the temperate latitudes. In its general aspect,
however, the Silurian fauna, antiquely fashioned, as I have said, as
became its place in the primeval ages of existence, was unlike any other
which the world ever saw; and the absence of the vertebrata, or at least
the inconspicuous place which they occupied if they were at all present,
must have imparted to the whole, as a group, a humble and mediocre
character. It seems to have been for many ages together a creation of
molluscs, corals, and Crustacea. At length, in an upper bed of the
system, immediately under the base of the Old Red Sandstone, the remains
of the earliest known fishes appear, blent with what also appears for
the first time,--the fragmentary remains of a terrestrial vegetation.
The rocks beneath this ancient bone-bed have yielded, as I have already
said, no trace of any plant higher than the Thallogens, or at least not
higher than the Zosteracea,--plants whose proper habitat is the sea;
but, through an apparently simultaneous advance of the two kingdoms,
animal and vegetable,--though of course the simultaneousness may be but
merely apparent,--the first land plants and the first vertebrates
appear together in the same deposit.

What, let us inquire, is the character of these ancient fishes, that
first complete the scale of animated nature in its four master ideas, by
adding the vertebrate to the invertebrate divisions? So far as is yet
known, they all consist of one well marked order,--that placoidal order
of Agassiz that to an internal framework of cartilage adds an external
armature, consisting of plates, spines, and shagreen points of solid
bone. Either of the two kinds of dog-fishes on our coasts,--the spiked
or spotted,--maybe accepted a