XI. NUMMULINIDACEAE.--As in Rotalidaceae, but with a thicker finely perforated shell, often well developed, and a supplementary skeleton traversed by branching canals as an additional party-wall between the proper chamber-walls. _Nonionina_ (d'Orb.) (fig. 4, 19); _Fusulina_ (Fischer) (fig. 20); _Polystomella_ (Lamk.) (figs. 4, 16; 8); _Operculina_ (d'Orb.) (fig. 9); _Heterostegina_ (d'Orb.) (fig. 16); _Cycloclypeus_ (Carptr.) (fig. 15); _Nummulites_ (Lamk.) (figs. 10, 11, 12, 13, 14). "_Eozoon canadense_," described as a species of this order by J.W. Dawson and Carpenter, has been pronounced by a series of enquirers, most of whom started with a belief in its organic structure, to be merely a complex mineral concretion in ophicalcite, a rock composed of an admixture of silicates (mostly serpentine and pyroxene) and calcite. _Distribution in Vertical Space._--Owing to their lack of organs for active locomotion the Foraminifera are all crawling or attached, with the exception of a few genera (very rich in species, however) which float near the surface of the ocean, constituting part of the pelagic plankton (q.v.). Thus the majority are littoral or deep-sea, sometimes attached to other bodies or even burrowing in the tests of other Foraminifera; most of the fresh-water forms are sapropelic, inhabiting the layer of organic debris at the surface of the bottom mud ditches of pools, ponds and lakes. The deep-sea species below a certain depth cannot possess a calcareous shell, for this would be dissolved; and it is in these that we find limesalts sometimes replaced by silica. [Illustration: FIG. 19.--Arenaceous Foraminifera. a, Exterior of _Saccammina_. b, The same laid open. c, Portion of test more highly magnified. d, _Pilulina_. e, Portion of test more highly magnified. f, Nautiloid _Lituola_, exterior. g, Chambered interior. h, Portion of labyrinthic chamber wall, showing component sand-grains.] [Illustration: FIG. 20.--Section of _Fusulina_ Limestone.] [Illustration: FIG. 21.--Microscopic Organisms in Chalk from Gravesend. a, b, c, d, _Textularia globulosa_; e, e, e, e, _Rotalia aspera_; f, _Textularia aculeata_; g, _Planularia hexas_; h, _Navicula_.]