Three theories have been proposed to account for the phenomenon: (1) that Orbulina is the terminal chamber of a Globigerina which has undergone complete involution, and is gradually absorbed; (2) that the Globigerina is developed within the Orbulina from a germ which has not been set free (Schacko, A. N. 49 (1), 1883, p. 437); (3) that the Orbulina is the initial, i. e. primordial chamber of a Globigerina, other chambers having been developed within it, and the case being one of dimorphism (Schlumberger, A. N. H. (5), xiv. 1884, p. 70). The figures and descriptions given by Shacko appear to be directly contradictory of Schlumberger's theory. Siddall has described curious disruptive phenomena in Shepheardella, which appear to be reproductive see his paper, Q. J. M. xx. 1880.
Carter has described, under the designation "TestamoebiformiaJ three organisms from the Gulf of Manaar (between South India and Ceylon), two adherent, creeping, with calcareous tests, and of these Holodadina is root-like, Cysteodictyina, retiform; the third Ceratestina, with subglobular chitinous chambers connected by stolons. Nothing is known of the living organism. See A. N. H. (5), v. 1880, p. 446.
The Dactyliporinae, which have been usually classed as Foraminifera, are now regarded as Calcareous Algae; Brady, 'Report,' etc, Challenger Reports, ix.
Structures known as Coccoliths and Rhabdoliths are generally found in Globigerina ooze and also fossilised in chalk. A Coccolith consists of a flat disc, or of two concave-convex discs, fitted into one another, composed of Calcium carbonate and an organic basis. A Rhabdolith with the same composition has a rod-like form, the actual shape of the rod varying much. Both occur associated in spherical masses, Coccospheres and Rhabdospheres, which are found floating at the surface of the ocean as well as at the bottom. By some they are regarded as Calcareous Algae; but it has been suggested that they are not unlike the forms assumed by calcareous matter when precipitated in an organic matrix. See Wallich, A. N. H. (3), viii. 1861; (4), xix. 1877; O. Schmidt, SB. Wien. Akad. lxii. Abth. 1, 1870; Carter, A. N. H. (4), vii. 1871; Wyville Thomson, 'Voyage of the Challenger,' The Atlantic, London, 1877, i. p. 220-2, figs. 49, 50.
It may be noted here that the term 'Foraminifera' was originally employed by D'Orbigny in allusion, not to the pores of the perforate test as is commonly supposed, but to the one or more pores in the divisions or septa between the chambers in the species known to him. D'Orbigny regarded the Foraminifera as a subdivision of Cephalopoda. See A. Sc. N. (I), vii. 1826, p. 245.