and overlap by “alary extensions,” becoming “nautiloid”; in extreme cases only the last turn or whorl is seen (fig. 11). When the spiral axis is conical the shell may be “rotaloid,” the larger lower chambers partially concealing the upper smaller ones (fig. 3, 12, 15, 17, 18); or they may leave, as in Patellina, a wide central conical cavity—which, in this genus, is finally occupied by later formed “supplementary” chambers. When the successive chambers are disposed around a longitudinal central axis they may be said to “alternate” like the leaves of a plant. If the arrangement is distichous we get such forms as Polymorphina, Textularia and Frondicularia (fig. 3, 13, 14), if tristichous, Tritaxia. Such an arrangement may coexist with a spiral twist of the axis for at least part of its course, as in the crozier-shaped Spiroplecta.
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Fig. 7.—Section of Rotalia beccarii, showing the canal system, a, b, c, in the substance of the intermediate skeleton; d, tubulated chamber-wall. |
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| Fig. 8.—Internal cast of Polystomella craticulata. |
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a, Retral processes, proceeding from the posterior margin of one of the segments. b, b¹, Smooth anterior margin of the same segment. c, c¹, Stolons connecting successive segments and uniting themselves with the diverging branches of the meridional canals. d, d¹, d², Three turns of one of the spiral canals. e, e¹, e², Three of the meridional canals. f, f¹, f², Their diverging branches. |
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| Fig. 9.—Operculina laid open, to show its internal structure. |
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a, Marginal cord seen in cross section at a’. b, b, External walls of the chambers. c, c, Cavities of the chambers. c′, c′, Their alar prolongations. d, d, Septa divided at d’, d’, and at d”, so as to lay open the interseptal canals, the general distribution of which is seen in the septa e, e; the lines radiating from e, e point to the secondary pores. g, g, Non-tubular columns. |
Two phenomena interfere with the ready availability of the characters of form for classificatory ends—dimorphism and multiformity.
Dimorphism.—The majority of foraminiferal shells show two types, the rarer with a much smaller central chamber than that of the more frequent. The chambers are called microsphere and megalosphere, the forms in which they occur microsphaeric and megalosphaeric forms, respectively. We shall study below their relation to the reproductive cycle.
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Fig. 10.—1, Piece of Nummulitic Limestone from the Pyrenees, showing Nummulites laid open by fracture through the median plane; 2, vertical section of Nummulite; 3, Orbitoides. |
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Fig. 11.—Vertical section of portion of Nummulites, showing the investment of the earlier whorls by the alar prolongations of the later. a, Marginal cord. b, Chamber of outer whorl. c, c, Whorl invested by a. d, One of the chambers of the fourth whorl from the margin. e, e′, Marginal portions of the enclosed whorls. f, Investing portion of the outer whorl. g, g, Spaces left between the investing portions of successive whorls. h, h, Sections of the partitions dividing these. |
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Fig. 12.—Internal surface of wall of two chambers, a, a, of Nummulites, showing the orifices of its minute tubuli. b, b, The septa containing canals. c, c, Extensions of these canals in the intermediate skeleton. d, d, Larger pores. |
Multiformity.—Many of the Polythalamia show different types of chamber-succession at different ages. We have noted this phenomenon in such crozier forms as Peneroplis, as well as in discoid forms; it is very frequent. Thus the microspheric Biloculina form the first few chambers in quinqueloculine succession. The microspheric forms attain to a greater size when adult than the megalospheric; and in Orbitolites the microsphere has a straight outlet, orthostyle, instead of the deflected camptostyle one, so general in porcellanous types; and the spiral succession is continued for more turns before reaching the fan-shaped and finally cyclic stage. Globigerina, whose chambers are nearly spherical, is sometimes seen to be enclosed in a spherical test, perforate, but without a pylome, and known as Orbulina; the chambered Globigerina-shell is attached at first inside the wall of the Orbulina, but ultimately disappears. The ultimate fate of the Orbulina shell is unknown; but it obviously marks a turning-point in the life-cycle.
Protoplasmic Body and Reproduction.—The protoplasm is not differentiated into ecto- and endosarc, although it is often denser
