At certain geological periods circumstances favoured the development of an enormous multitude of individual animals. In the earlier part of the Tertiary period the Nummulites acquired an extraordinary size. They were like very large coins two or more inches in diameter, and were accumulated in such quantities as to constitute the chief part of the nummulitic limestone; a formation in some places 1,500 feet thick, which extends through southern Europe, Libya, Egypt, Asia Minor, and is continued through the Himalayan mountains into various parts of the Indian peninsula, where it is extensively distributed. The Great Pyramid of Egypt is built of this limestone, which gave rise to singular speculations with regard to the Nummulites in very ancient and even in more recent times. Although this is incomparably the greatest, it is by no means the only instance of an accumulation of the fossil shells of individual animals. The ‘Lingula flags,’ a stratum in the upper Cambrian series of North Wales, was so named from the abundance of the Brachiopod Lingula that it contains. Professor Ehrenberg discovered that the shells of the Foraminifera sometimes undergo an infiltration of silicate of iron, which fills not only the chambers, but also their canal-system even to its minutest ramifications, so that if the shell be destroyed by dilute acid, a perfect cast of the sarcode matter remains. The greensands in the different geological strata from the Silurian formation upwards, are chiefly composed of these casts; and Professor Baily of the United States more recently discovered that a process of infiltration is even now taking place in some parts of the ocean bed, and that beautiful casts of Foraminifera may be obtained by dissolving their shells with dilute acid. A most extensive comparison of the Foraminiferous group of Rhizopods, recent and fossil, has been made by Messrs. Parker and Rupert Jones from almost every latitude on the globe, from the arctic and tropical seas, from the temperate zones in both hemispheres, and from shallow as well as deep-sea beds. They have also reviewed the fossil Foraminifera in their manifold aspects as presented by the ancient geological faunas throughout the whole series from the Tertiary down to the Carboniferous strata inclusive; and have come to the astonishing conclusion that scarcely any of the species of the Foraminifera met with in the secondary rocks have become extinct. All that they had seen have their counterparts in the recent Mediterranean deposits. Throughout that long series of geological epochs even to the present day, the Foraminifera show no tendency to rise to a higher type; but variety of form in the same species prevailed then as it does now. Subsequently to this investigation, a gigantic Orbitulite twelve inches in diameter, and the third of an inch thick, has been found in the Silurian strata in Canada. The largest recent species Dr. Carpenter had seen was about the size and thickness of a shilling.