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. [Illustration: Fig. 116--Corals of the Carboniferous Limestone. a. _Cyathophyllum paracida_, showing young corallites budded forth from the disc of the old one; a', One of the corallites of the same, seen in cross-section; b, Fragment of a mass of _Lithostrotion irregulare_; b', One of the corallites of the same, divided transversely; c, Portion of the simple cylindrical coral of _Amplexus coralloides_; c', Transverse section of the same species; d, _Zaphrentis vermicularis_, showing the depression or "fossula" on one side of the cup; e, Fragrent of a mass of _Syringopora ramulosa_; f, Fragment of _Coetetes tumidus_; f', Portion of the same of the same, enlarged. From the Carboniferous Limestone of Britain and Belgium. (After Thomson, De Koninck, Milne-Edwards and Haime, and the Author.)]