[Illustration: 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.] [Illustration: 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_. [Illustration: FIG. 7.--Section of _Rotalia beccarii_, showing the canal system, a, b, c, in the substance of the intermediate skeleton; d, tubulated chamber-wall.] [Illustration: FIG. 8.--Internal cast of _Polystomella craticulata_. a, Retral processes, proceeding from the posterior margin of one of the segments. b, b^1, Smooth anterior margin of the same segment. c, c^1, Stolons connecting successive segments and uniting themselves with the diverging branches of the meridional canals. d, d^1, d^2, Three turns of one of the spiral canals. e, e^1, e^2, Three of the meridional canals. f, f^1, f^2, Their diverging branches.] [Illustration: 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.]