[Illustration: Fig. 17. Nummulitis lenticularis.] [Illustration: Fig. 18. Cassidulina.] [Illustration: Fig. 19. Textilaria.] [Illustration: Fig. 20. Spiroloculina.] In the fourth group the shell is spiral, with the chamber equilateral, with a larger and smaller side, the position being alternately reversed as the segments are multiplied, as in _Cassidulina_ (Fig. 18). In the succeeding group the new segments are arranged alternately on opposite sides of the central line, as in _Textilaria_ (Fig. 19), thus forming two alternating non-spiral parallel segments, each connected by a single orifice. The sixth family differ entirely in appearance and structure from the other Foraminifera. They are more opaque than the other orders, having a resemblance to white porcelain, which presents a rich amber-brown hue when viewed by transmitted light. They are more or less oblong, each new segment being nearly equal to the entire length of the shell, so that the terminal orifice presents itself alternately at its opposite extremities, sometimes in one uniform plane, as in _Spiroloculina_ (Fig. 20), and _Faujasina_ (Fig. 21). At other times each new segment, instead of being exactly opposite each other, is a little on one side. [Illustration: Fig. 21. Faujasina.] Professor Williamson has shown that the shell enclosing each new segment is at first very thin; but as additional calcareous chambers are formed, each addition not only encases the new gemmation of the soft animal, but extends over all the exterior of the previously formed shell. The exact manner in which this is accomplished is doubtful; but the Professor thinks it probable that the soft animal has the power of diffusing its substance over the shell, and thus depositing upon its surface additional layers of calcareous matter. The fossil Foraminifera are chiefly distinguished from recent and existing species by the size of the former. While the living forms range from one-fourth to the one-hundredth part of an inch, the tertiary strata abound in examples of _Nummulites_ varying from the eighth of an inch to the size of half-a-crown. The engraving is a drawing from Nature, by MM. d'Archaic and Haime, of a piece of nummulitic rock, of Nousse, in the Landes, in which a great variety of sizes and forms are exhibited. The Nummulina belong to the third family, or Helicostega, in which the outer convolutions completely embrace the earlier-formed ones. Hence it is only by making microscopic sections, or thin slices, that their structure can be fully seen. When such a section is carried horizontally through the centre of the shell, the segments present a spiral arrangement, which, like the convolutions, are remarkable for their small size, and consequent great number. [Illustration: Fig. 22. Nummulites Rouaulti (d'Archaic and J. Haime).] [Illustration: Fig. 23. Siderolites calcitrapoides (Lamarck). Natural size and magnified.]