5. Segments developed alternately on the right and left of the first, and successively on each side the longitudinal axis; as in Textularia, [Lign. 109].
6. Arrangement of the segments combining the modes of 4 and 5; that is, the segments are formed alternately, but the whole are coiled spirally, either obliquely, or on the same plane; as in Amphistigena.
7. Segments round a common axis, on two, three, four, or five, opposite faces, returning after each entire revolution; the new cells being placed exactly on the preceding series; as in Quinqueloculina, Pict. Atlas, pl. lxii. fig. 12.
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.
Lign. 110. Nummulites, or Nummulina; nat.
From the Great Pyramid of Egypt.