ey are invariably confined to special horizons
in the strata, and they indicate temporary depressions of the
land beneath the sea. Whilst the distinction here mentioned is
one which cannot fail to strike the observer, it is convenient
to consider the animal life of the Carboniferous as a whole: and
it is simply necessary, in so doing, to remember that the marine
fossils are in general derived from the inferior portion of the
system; whilst the air-breathing, fresh-water, and brackish-water
forms are almost exclusively derived from the superior portion
of the same.

[Illustration: Fig. 114.--Transparent slice of Carboniferous
Limestone, from Spergen Hill, Indiana, U.S., showing numerous
shells of _Endothyra_ (_Rotalia_), _Baiteyi_ slightly enlarged.
(Original.)]

[Illustration: Fig. 115.--_Fusulina cylindrica_, Carboniferous
Limestone, Russia.]

The Carboniferous _Protozoans_ consist mainly of _Foraminifera_
and _Sponges_. The latter are still very insufficiently known,
but the former are very abundant, and belong to very varied types.
Thin slices of the limestones of the period, when examined by the
microscope, very commonly exhibit the shells of _Foraminifera_
in greater or less plenty. Some limestones, indeed, are made up of
little else than these minute and elegant shells, often belonging
to types, such as the Textularians and Rotalians, differing little
or not at all from those now in existence. This is the case, for
example, with the Carboniferous Limestone of Spergen Hill in
Indiana (fig. 114), which is almost wholly made up of the spiral
shells of a species of _Endothyra_. In the same way, though to a
less extent, the black Carboniferous marbles of Ireland, and
the similar marbles of Yorkshire, the limestones of the west
of England and of Derbyshire, and the great "Scar Limestones" of
the north of England, contain great numbers of Foraminiferous
shells; whilst similar organisms commonly occur in the shale-beds
associated with the limestones throughout the Lower Carboniferous
series. One of the most interesting of the British Carboniferous
forms is the _Saccammina_ of Mr Henry Brady, which is sometimes
present in considerable numbers in the limestones of Northumberland,
Cumberland, and the west of Scotland, and which is conspicuous
for the comparatively large size of its spheroidal or pear-shaped
shell (reaching from an eighth to a fifth of an inch in size).
More widely distributed are the generally spindle-