| Fig. 5.—Shell of simple type of Orbitolites, showing primordial chamber a, and circumambient chamber b, surrounded by successive rings of chamberlets connected by circular galleries which open at the margin by pores. |
| Fig. 6.—Animal of simple type of Orbitolites, showing primordial segment a, and circumambient segment b, surrounded by annuli of sub-segments connected by radial and circular stolon-processes. |
The Cheilostomellidae (fig. 3, 13) reproduce among perforate vitreous genera what we have already seen in the Miliolida: Orbitoides (fig. 10, 2) and Cycloclypeus, among the Nummulite group, with a very finely perforate wall, recall the porcellanous Orbiculina and Orbitolites.
In flat spiral forms (figs. 22, 1, 7; 3, 2, 16, 19, &c.) all the chambers may be freely exposed; or the successive chambers be wider transversely than their predecessors 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.
| Fig. 7.—Section of Rotalia beccarii, showing the canal system, a, b, c, in the substance of the intermediate skeleton; d, tubulated chamber-wall. |
 | |
| Fig. 8.—Internal cast of Polystomella craticulata. | |
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. |
 | |
| Fig. 9.—Operculina laid open, to show its internal structure. | |
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.
| 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. |