There are two epochs of the earth's history in which foraminifera were remarkable for their size as well as their abundance. The first gave us the grey Fusulina limestone of Upper Carboniferous times, when this spindle-shaped shell spread freely from the United States through the arctic regions to the east of Asia. The second gave us, in the Eocene period, the great beds formed of Nummulites and Orbitoides, which we meet with in Europe on the Lake of Thun, but which are far more important in Lower Egypt. The disc-like forms of the nummulites in the white limestone of the Pyramids are familiar to hundreds of travellers, and forms are recorded up to four and a half inches across. The foraminiferal origin of many compact limestones can often be appreciated on smooth surfaces with a pocket-lens. The older examples have commonly become stained and darkened, and crystallisation of calcite throughout the ground has in part destroyed the original organic structures. This tendency to crystallise affects even the larger fossils, and brachiopods and molluscs have sometimes disappeared from our Carboniferous limestones, without the intervention of "metamorphic" heat or pressure. In most limestones older than the Eocene period, the shells and other fossils, such as corals, that were originally formed of aragonite have passed into the calcite state, without the destruction of their characteristic shapes. Shells, however, have been found still preserved as aragonite in beds as old as the Jurassic period[8]. The lamellibranchs, the ordinary bivalves, came into prominence as limestone-builders with the Carboniferous period, and are now rivalled by the univalve gastropods, which displayed no widespread activity until Eocene times. The most massive existing shell, however, is a lamellibranch, the giant Tridacna of Australian seas, a single valve of which may weigh 250 lbs. The cephalopods, though lying far nearer to the crown of molluscan development, became important from the Silurian Orthoceras onwards, and nautiloids of various forms are common fossils in the Carboniferous limestone. Their large size attracts attention from our present point of view. The cephalopods, however, swell the bulk of many limestones, not by the thickness of their shells, but through their chambered character, which has prevented complete infilling of the shell, and which thus allows of cavities in the mass. This is notably the case with the ammonites, which contribute so largely to Jurassic limestones. Crystalline calcite has often been deposited by infiltration on the septa and on the inner layer of the shell, thus reducing the hollow spaces. The massive calcite guards of the belemnites form a considerable part of many limestones.