fresh or brackish waters.  Marine fossils may be found in the Coal-measures, but they 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-shaped shells of Fusulina (fig. 115), which occur in vast numbers in the Carboniferous Limestone of Russia, Armenia, the Southern Alps, and Spain, similar forms occurring in equal profusion in the higher limestones which are found in the Coal-measures of the United States, in Ohio, Illinois, Indiana, Missouri, &c.  Mr Henry Brady, lastly, has shown that we have in the Nummulina Pristina of the Carboniferous Limestone of Namur a genuine Nummulite, precursor of the great and important family of the Tertiary Nummulites.