As regards the animals of the Eocene period, the Protozoans are represented by numerous Foraminifera, which reach here their maximum of development, both as regards the size of individuals and the number of generic types.  Many of the Eocene Foraminifers are of small size; but even these not uncommonly form whole rock-masses.  Thus, the so-called “Miliolite Limestone” of the Paris basin, largely used as a building-stone, is almost wholly composed of the shells of a small species of Miliola.  The most remarkable, however, of the many members of this group of animals which flourished in Eocene times, are the “Nummulites” (Nummulina), so called from their resemblance in shape to coins (Lat. nummus, a coin).  The Nummulites are amongst the largest of all known Foraminifera, sometimes attaining a size of three inches in circumference; and their internal structure is very complex (fig. 214).  Many species are known, and they are particularly characteristic of the Middle and Upper of these periods—­their place being sometimes taken by Orbitoides, a form very similar to the Nummulite in external appearance, but differing in its internal details.  In the Middle Eocene, the remains of Nummulites are found in vast numbers in a very widely-spread and easily-recognised formation known as the “Nummulitic Limestone” (fig. 10).  According to Sir Charles Lyell, “the Nummulitic Limestone of the Swiss Alps rises to more than 10,000 feet above the level of the sea, and attains here and in other mountain-chains a thickness of several thousand feet.  It may be said to play a far more conspicuous part than any other Tertiary group in the solid framework of the earth’s crust, whether in Europe, Asia, or Africa.  It occurs in Algeria and Morocco, and has been traced from Egypt, where it was largely quarried of old for the building of the Pyramids, into Asia Minor, and across Persia by Bagdad to the mouths of the Indus.  It has been observed not only in Cutch, but in the mountain-ranges which separate Scinde from Persia, and which form the passes leading to Cabul; and it has been followed still further eastward into India, as far as Eastern Bengal and the frontiers of China.”  The shells of Nummulites have been found at an elevation of 16,500 feet above the level of the sea in Western Thibet; and the distinguished and philosophical geologist just quoted, further remarks, that “when we have once arrived at the conviction that the Nummulitic formation occupies a middle and upper place in the Eocene series, we are struck with the comparatively modern date to which some of the greatest revolutions in the physical geography of Europe, Asia, and Northern Africa must be referred.  All the mountain-chains—­such as the Alps, Pyrenees, Carpathians, and Himalayas—­into the composition of whose central and loftiest parts the Nummulitic strata enter bodily, could have had no existence till after the Middle Eocene period.  During that period, the sea prevailed where these chains now rise; for Nummulites and their accompanying Testacea were unquestionably inhabitants of salt water.”