They may in some instances be subdivided into chamberlets by vertical partitions, imperfect in the imperforate Orbiadina, Orbitolites, Alveolina, perfect in the perforate Cycloclypeinae. In recent species of Alveolitia the chamberlets in turn are subdivided. The septa dividing the chambers in the imperforate Foraminifera and the simpler perforate, are single and formed by a portion of the outer wall of the preceding chamber; in the higher perforate forms, e. g. larger species of Rotalia, in Polystomella, they are double, a second layer being added to the surface of the old chamber at the formation of a new chamber. The first, primordial, or embryonal chamber differs from its successors; it is globular, ovate or lens-like. The difference is naturally most marked in complicated shells. In some Rotalidae and Nummu-linidae there is a supplemental, exogenous, or secondary skeleton in addition to the chamber walls proper. This secondary deposit may fill up the depressions between successive chambers and the central umbilical cavities due to the increasing size of successive coils, or it may coat the entire test as in Calcarina and Heterostegina. Small projecting masses of it are found between the edges of the chamberlets in the Cycloclypeinae. It may grow out into spines between the chambers of the larger species of Rotalia, or from the surface in general as in Calcarina. A system of more or less complicated 'interseptal' canals opening superficially is usually developed in connection with this skeleton between the convolutions and septa: by its means the protoplasm of the more deeply situated parts communicates with the surface.
The surface of the test may be ornamented with pits, areolae, ridges or bands; it is sometimes spinulose, in Globigerinidae especially, the spines attaining a great length and being moveable in Hastigerina. With few exceptions the test itself is colourless; it is red in Polytrema and Globigerina rubra.
1Gruber has described an entosolenian Gromia Lagenoides; Nova Acta, xlvi. p. 495.
An adventitious skeleton is characteristic of the two families Astro-rhizidae and Lituolidae. The material of which it is composed is selected and is in most instances sand-grains, which are loose with very little cement indeed in Astrorhizinae, in others generally cemented firmly together. The nature of the cement varies; it is chitinous, and the test flexible in Rhizammina or brittle in Rheophax membranacea; as a rule it consists of Ferric oxide and Calcium carbonate in varying proportions, the former being especially predominant in some Lituolidae, viz. 16-3 per cent, in Haplophragmium latidorsatum, and 9-4 per cent, in Cyclammina cancellata. It is very rarely siliceous, e. g. in Rheophax nodidosa. The surface of the test is generally rough, but in the Trochamtnininae it is smooth and polished, being composed of a large amount of cement with very fine sand-grains. Mud coating a chitinoid membrane is found in the Astrorhizine Pelosina; so too in Dendrophrya, but the chitinoid membrane is beset with sand-grains. Sponge-spicules felted together, mixed with fine sand but not united by cement, characterise the test of the Pilidininae. Sponge-spicules are also preferentially selected by Haliphysema mixed with other foreign bodies and united by a calcareous cement; they are used also by some Lituolinae. The Trochammine Carterina is unique in forming proper fusiform calcareous spicules which with sand-grains and a calcareous cement make up its Rotaline test.
The character of the sea-bottom often influences the composition of the test. Foraminiferal tests are used where Globigerina ooze prevails; the shells of Radiolaria, frustules of Diatoms where they abound to the exclusion of other material. Ammodiscus incertus, from a bottom of Radiolarian ooze, has a siliceous cement; Trochammina inflata loses it calcareous cement and becomes chitino-arenaceous when living in brackish water, and in the variety T. macrescens, the test is a flexible membrane with scarcely any calcareous investment.
As to shape, the Lituolidae are for the most part isomorphous with calcareous Foraminifera; even Loftusia finds a representative in Alveolina and Parkeria in Keramosphaera. With the exception of some Endo-thyrinae, an extinct sub-family, all are imperforate. The test is sometimes monothalamous, e. g. some species of Rheophax, Thtirammina, etc, but more generally polythalamous. The septa are often imperfect in some Lituolinae, e. g. in Lituola they are labyrinthic. The walls of the test in the Loftusinae, e. g. Cyclammina, are thick and cancellated, i. e. traversed by passages, with the exception of a thin superficial layer. Some forms occur both free and adherent; some are always adherent. Among the latter Webbina, which has either a simple spherical chamber or an oval chamber with a tube, or several oval chambers connected by tubes, has no wall on the attached side. The Astrorhizidae present peculiar types of shape. Psammosphaera is a globe with interstitial apertures here and there, Sorospkaera, a collection of such globes; in Storthosphaera the orifices are slightly tubulated.
Globular or oval forms with a single distinct aperture are seen in Sac-camina, Pelosina, Pihdina, Technitella. The first-named occurs sometimes in groups, the members of which may be connected by tubular stolons. Or the test is a tube closed at one end which may or may not be dilated into a chamber, straight in Iacidella, sinuous or branched and sometimes adherent in Hyperammina. A simple tube, open at each end, is seen in some species of Astrorhiza, in Bathysiphon and Marsipella. In Rhabdam-mina there is a central chamber, sometimes scarcely marked, with two or more tubular arms typically radiating from it. This central chamber becomes large, the arms more or less numerous, irregular in shape, and sometimes branching, in some species of Astrorhiza and in Aschemonella; in the latter several chambers may be connected together irregularly by tubes. Dendrophrya has the chamber adherent, the arms branched, spreading or erect. Syringammina consists of a rounded mass of branching sand-tubes, radiating from a common centre and connected at intervals by lateral branches; the terminal apertures of the tubes are filled with loose sand-grains. The extinct Syringosphaera and Stoliczkaria have a similar structure.