The Foraminifera vary considerably in magnitude; by far the greater number of species are invisible to the unassisted eye, and the aid of a lens or microscope is required to define the structure even of the largest; yet many are of sufficient size to be recognized, as for example the _Spirolinæ_, _Lign. 112_. A few genera are from a quarter of an inch to nearly an inch in diameter, as the _Orbitoides Mantelli_ of the tertiary formations of North America,[312] and the _Nummulina_, commonly termed _Nummulites_, of Europe, _Lign. 110_. We will now describe the genera selected for illustration, commencing with the large and well-known type, whose aggregated remains form extensive beds of crystalline limestone in the Alps, and in Asia, and Africa.[313] [312] Dr. Morton's Synopsis of the Organic Remains of the Cretaceous group of the United States. Philadelphia, 1824, p. 45, pl. v. fig. 9. [313] Geol. Journal, vol. v. [Illustration: Lign. 110. Nummulites, or Nummulina; _nat._ _From the Great Pyramid of Egypt._ Fig. 1.--Transverse section of a Nummulite, showing the form and arrangement of the cells. 2, 3.--Specimens with part of the external plate removed. ] Nummulina lævigata. _Lign. 110._--The shell is of a discoidal or lenticular form, composed of numerous cells, concentrically arranged round an axis on the same plane; both sides of the disk covered by a smooth thick plate. [Sidenote: NUMMULITES.] Under the name of _Nummulites_, from their resemblance to a piece of money, the fossil shells of this genus of Foraminifera have long been known to naturalists, and are figured in many of the early works on petrifactions. They occur in immense quantities in certain rocks, and are of all sizes, from a mere point, to disks an inch and a half in diameter; thus exceeding in magnitude all other animals of this class. Perfect specimens appear as a calcareous solid circular body, of a lenticular shape; smooth, and slightly convex on both sides, and without any visible structure. On splitting the fossil transversely, or rubbing down one of the convex planes, a series of minute cells, arranged in a discoidal spire, is brought to view, as shown in _Lign. 110, fig. 1_. But this description gives a very inadequate idea of the complicated and exquisite structure of the original, which has been admirably worked out by Dr. Carpenter. This eminent physiologist has shown that each chamber was occupied by a living segment, connected with other segments by one or more tubular prolongations, which absorbed nutrition from without, by means of filamentous pseudopodia, that projected through a system of passages leading from the medial plane to the external surface.[314] A figure of the supposed form of a living Nummulina is given in Pict. Atlas, p. 187.