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. [Illustration: 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_.] [Illustration: FIG. 11.--Vertical section of portion of _Nummulites_, showing the investment of the earlier whorls by the alar prolongations of the later. a, Marginal cord. b, Chamber of outer whorl. c, c, Whorl invested by a. d, One of the chambers of the fourth whorl from the margin. e, e', Marginal portions of the enclosed whorls. f, Investing portion of the outer whorl. g, g, Spaces left between the investing portions of successive whorls. h, h, Sections of the partitions dividing these.] [Illustration: FIG. 12.--Internal surface of wall of two chambers, a, a, of _Nummulites_, showing the orifices of its minute tubuli. b, b, The septa containing canals. c, c, Extensions of these canals in the intermediate skeleton. d, d, Larger pores.] _Multiformity._--Many of the Polythalamia show different types of chamber-succession at different ages. We have noted this phenomenon in such crozier forms as _Peneroplis_, as well as in discoid forms; it is very frequent. Thus the microspheric _Biloculina_ form the first few chambers in quinqueloculine succession. The microspheric forms attain to a greater size when adult than the megalospheric; and in _Orbitolites_ the microsphere has a straight outlet, orthostyle, instead of the deflected camptostyle one, so general in porcellanous types; and the spiral succession is continued for more turns before reaching the fan-shaped and finally cyclic stage. _Globigerina_, whose chambers are nearly spherical, is sometimes seen to be enclosed in a spherical test, perforate, but without a pylome, and known as _Orbulina_; the chambered Globigerina-shell is attached at first inside the wall of the _Orbulina_, but ultimately disappears. The ultimate fate of the _Orbulina_ shell is unknown; but it obviously marks a turning-point in the life-cycle.