CHALK FORAMINIFERA.
Lign. 115. Chalk-dust; chiefly composed of Foraminifera; highly magnified.
(Seen by reflected light.)
a, a. Rotaliæ. b. Textulariæ.
Foraminifera of the Chalk and Flint. [Lign. 115.]—If a few grains of soft white chalk from Gravesend or Dover be examined under a high power (1/4 inch object-glass of Ross), groups of foraminifera will be perceived, chiefly of Rotaliæ, Rosalinæ, and Textulariæ, as shown in [Lign. 115]. If some of the powder be immersed in Canada balsam, (as directed in the instructions at the close of this chapter for preparing chalk for microscopical examination,) the outline of the shells, and the cavities of, the cells, will be apparent; as in the sketches [Lign. 114], ante [p. 351]. If a chip or slice of flint, rendered transparent by immersion in oil of turpentine or Canada balsam, be viewed first with a low power, (1 inch object-glass,) to discover a good specimen, and afterwards under a high magnifier, (a 1/4 or an 1/8 object-glass,) the form of the shell and of the cells will be distinctly seen, as in [Lign. 116]. In this beautiful fossil Rotalia, the segments are as sharply defined as in a recent example: and one of the cells (a) is seen to be lined with quartz crystals.
Lign. 116. Section of a Rotalia in flint.
(Seen by transmitted light, and showing the chambers partially filled with mineral matter.)
a. A cell lined with spar.
(× 250 diameters.)
At first sight this fossil might be mistaken for a nautilus partially filled with spar; but the reader will remark that the septa, or partitions, have their convex surface towards the aperture; whereas in the shells of the Cephalopoda (Nautilus, Ammonite, &c.) the septa are concave anteriorly. In [Lign. 114, fig. 4], a series of casts in flint of the septa of a young Nautilus is represented; by comparing it with the Rotaliæ in the same lignograph, figs. 2, 3, this distinction will be obvious. And here it may be necessary again to point out the essential character of the animal of the foraminifera, as distinguished from that of the cephalopoda with chambered shells. In the latter, the body of the mollusk only occupies the large outer chamber; the internal compartments are empty dwellings, which the animal has successively quitted in the progress of its growth, and with which it has no connexion except by the siphunculus. In the Rotalia, and allied forms, the body of the animal is inclosed within the shells, and occupies every chamber contemporaneously at every stage: the cells are always filled by the segments of the body. Hence when the shell, which is calcareous, is dissolved in weak hydrochloric acid, the soft body is exposed, and seen to extend to the innermost chamber. The segments are connected by a membranous tube, which some naturalists regard as a common channel of communication between the several digestive sacs of which the body consists; for minute diatomaceæ which the animal has swallowed, (according to Ehrenberg,) are seen within the membranous sacs; as shown in [Lign. 113], which represents the body of a Nonionina, deprived of its shell. The importance of obtaining a correct idea of this structure will presently appear.
When a recent Rotalia is immersed in dilute acid, the soft parts of the body, deprived of the shell, may be obtained entire; they consist of a series of little bags or sacs, united by a tube. The constituent substance appears to be a tough membrane, and is generally of a rich brown or amber colour. The sacs are sometimes full of a granular substance, but are often empty and collapsed.