Remains of Foraminifera have been found in Palaeozoic, Mesozoic, and Kainozoic formations. In the oldest stratified rocks with which we are acquainted - viz., the Laurentian rocks of Canada - there occurs a singular body which has been described as the remains of a gigantic Foraminifer, under the name of Eozoon Canadense. If truly organic, as is doubted by high authorities, it is the oldest fossil as yet discovered. It appears to have grown in reef-like masses resembling the sessile patches of Polytrema* and Calcarina, to both of which, as well as to the extinct Nummulites, it shows a decided affinity. In the Silurian rocks, remains of Foraminifera, some of which are apparently identical with existing genera, have been detected in various places, and it is not impossible that the large Silurian fossils known as Receptaculites and Stromatopora should really be referred to this order. Little is yet known of the Foraminifera of the Devonian period; but the remains of these organisms are found abundantly in the Carboniferous, and less plentifully in the Permian deposits. Whole beds of the Carboniferous Limestone in Russia, Armenia, N. America, etc, are made up of the shells of Fusulina; and in Britain Mr Brady has shown that the same formation is occasionally largely composed of the arenaceous spheres of Saccammina, a genus which is especially interesting, as Sars has found vast numbers of a living form at considerable depths in the North Sea, and as it is known to occur in rocks as old as the Lower Silurian. In the Secondary rocks Foraminifera occur in great abundance, the widely spread formation known as the Chalk being crowded with these organisms. Chalk itself, in fact, is very largely composed of the cases of Foraminifera, some of which are identical with species now existing.
In the Tertiary rocks the Foraminifera attain their maximum of development, both as regards the size and the number of the forms which characterise them. The period of the Middle Eocene is especially distinguished by a very widely spread and easily recognised rock known as the Nummulitic Limestone, so called from the abundance in it of a large coin-shaped Fora-minifer termed the Nummulite (fig. 16). The Nummulitic Limestone stretches from the west of Europe to the frontiers of China; but in some cases, in place of Nummulina proper, it contains the remains of a mimetic form termed Orbitoides. Upon the whole, Dr Carpenter concludes that "there is no evidence of any fundamental modification or advance of the foraminiferous type from the Palaeozoic period to the present time."
* Polytrema is a little branched coral-like Foraminifer, composed of a calcareous test forming a number of irregular chambers, which communicate with one another by wide orifices, and are filled with colourless sarcode. The walls of the chambers are also penetrated by an extensive system of capillary canals.
Fig. 16. - Nummulina Icevigata. Eocene.