tical with any living fish; and
he adds the important remark that even in the lower Tertiary formations
a third of the fossil fishes of the _calcaire grossier_ and of the
London clay belong to extinct families.

We have seen that fishes, which are the oldest vertebrates, first appear
in the Silurian strata, and are found in all the succeeding formations
up to the birds of the Tertiary Period. Reptiles begin in like manner in
the magnesian limestone, and if we now add that the first mammalia are
met with in Oolite, the Stonefield slate; and that the first remains of
birds have been found in the deposits of the cretaceous period, we shall
have indicated the inferior limits, according to our present knowledge,
of the four great divisions of the vertebrates.

In regard to invertebrate animals, we find corals and some shells
associated in the oldest formations with very highly organised
cephalopodes and crustaceans, so that widely different orders of this
part of the animal kingdom appear intermingled; there are, nevertheless,
many isolated groups belonging to the same order in which determinate
laws are discoverable. Whole mountains are sometimes found to consist of
a single species of fossil goniatites, trilobites, or nummulites.

Where different genera are intermingled, there often exists a
systematic relation between the series of organic forms and the
superposition of the formations; and it has been remarked that the
association of certain families and species follows a regular law in the
superimposed strata of which the whole constitutes one formation. It has
been found that the waters in the most distant parts of the globe were
inhabited at the same epochs by testaceous animals corresponding, at
least in generic character, with European fossils.

Strata defined by their fossil contents, or by the fragments of other
rocks which they include, form a geological horizon by which the
geologist may recognise his position, and obtain safe conclusions in
regard to the identity or relative antiquity of formations, the
periodical repetition of certain strata--their parallelism--or their
entire suppression. If we would thus comprehend in its greatest
simplicity the general type of the sedentary formations, we find in
proceeding successively from below upwards: (1) The Transition group,
including the Silurian and Devonian (Old Red Sandstone) systems; (2) the
Lower Trias, comprising mountain limestone, the coal measures, the lo