[314] Geol. Journal, vol. vi. p. 21. See also a paper by Prof. Williamson, "On the minute Structure of the Calcareous Shells of some recent species of Foraminifera." Trans. Microscop. Soc. vol. iii. p. 105. The specimens figured in _Lign. 110_ are from the limestone that forms the foundation rock of the Great Pyramid of Egypt, and of which that structure is in great part composed. Strabo alludes to the Nummulites of the Pyramids, as lentils which had been scattered about by the artificers employed on those stupendous monuments, and become stone.[315] Silicified masses of Nummulites are occasionally met with; polished slices of such specimens are richly figured by the sections of the inclosed Foraminifera. [315] An interesting fact was communicated to me by a friend who lately descended the Nile; the Nummulitic limestone rocks are in some parts of the course of the river washed into the stream, and becoming disintegrated, the Nummulites are set free, and re-deposited in the recent mud of the Nile. The Nummulitic limestones are of the Eocene or ancient. Tertiary epoch, as the labours of Sir Roderick Murchison in the Alps, Apennines, and Carpathians first established: Nummulites are unknown in the Secondary formations.[316] [316] Geol. Journal, vol. v. Orbitoides.--The fossil bodies thus named are disciform, like the Nummulites; and one species, which forms the constituent substance of ranges of limestone mountains, 300 feet high, near Suggsville, in North America, was first described by my lamented friend, the late Dr. Morton, of Philadelphia, as _N. Mantelli_, in his work on the Cretaceous Fossils of the United States. The discovery that the Nummulitic deposits of the Old World were of the tertiary period, directed attention to their supposed geological equivalents in America; and on a careful examination of their fossils, the rocks proved to be tertiary, and the shells true Foraminifera allied to the Nummulites, but generically distinct.[317] A reference to Dr. Carpenters memoir, previously cited, is necessary to comprehend the complicated structure of these fossils.[318] [317] Manual of Geology, p. 208. [318] Geol. Journal, vol. vi. Siderolina, or _Siderolites_, is a genus of Foraminifera, which may be described as Nummulites, in which the turns of the spire are intercepted by elongated appendages, that project beyond the periphery of the disk, and produce a stellated figure. These fossils abound in the cretaceous strata of Maestricht. Fusulina.--The shell is fusiform, being elongated transversely to the axis; the cells are divided internally by constrictions. Only one species is known, (_F. cylindrica_,) and this is confined to the Carboniferous formation; it is the most ancient or earliest type of the class, according to the present state of our knowledge. [Illustration: Lign. 111. Foraminifera, &c.