groups of the Annulosa, the Insecta and the Arachnida,
are represented in the Coal, either by existing genera, or by forms
differing from existing genera in quite minor peculiarities.

Turning to the Vertebrata, the only palaeozoic Elasmobranch Fish of which
we have any complete knowledge is the Devonian and Carboniferous
_Pleuracanthus_, which differs no more from existing Sharks than these do
from one another.

Again, vast as is the number of undoubtedly Ganoid fossil Fishes, and
great as is their range in time, a large mass of evidence has recently
been adduced to show that almost all those respecting which we possess
sufficient information, are referable to the same sub-ordinal groups as
the existing _Lepidosteus, Polypterus_, and Sturgeon; and that a singular
relation obtains between the older and the younger Fishes; the former,
the Devonian Ganoids, being almost all members of the same sub-order as
_Polypterus_, while the Mesozoic Ganoids are almost all similarly allied
to _Lepidosteus_.[5]

[Footnote 5: "Memoirs of the Geological Survey of the United Kingdom.--
Decade x. Preliminary Essay upon the Systematic Arrangement of the Fishes
of the Devonian Epoch."]

Again, what can be more remarkable than the singular constancy of
structure preserved throughout a vast period of time by the family of the
Pycnodonts and by that of the true Coelacanths; the former persisting,
with but insignificant modifications, from the Carboniferous to the
Tertiary rocks, inclusive; the latter existing, with still less change,
from the Carboniferous rocks to the Chalk, inclusive?

Among Reptiles, the highest living group, that of the Crocodilia, is
represented, at the early part of the Mesozoic epoch, by species
identical in the essential characters of their organisation with those
now living, and differing from the latter only in such matters as the
form of the articular facets of the vertebral centra, in the extent to
which the nasal passages are separated from the cavity of the mouth by
bone, and in the proportions of the limbs.

And even as regards the Mammalia, the scanty remains of Triassic and
Oolitic species afford no foundation for the supposition that the
organisation of the oldest forms differed nearly so much from some of
those which now live as these differ from one another.

It is needless to multiply these instances; enough has been said to
justify the statement that, in view of the immense diversity of known
anim