the chromidial mass with the micronucleus, whose chief function is reproductive. [Illustration: FIG. 14.--Vertical section of tubulated chamber-walls, a, a, of _Nummulites_. b, b, Marginal cord; c, cavity of chamber; d, d, non-tubulated columns.] Since megalospheric forms are by far the most abundant, it seems probable that under most conditions they also give rise to megalospheric young like themselves; and that the production of zoospores, pairing to pass into the microspheric form, is only occasional, and possibly seasonal. This life-history we owe to the researches of Schaudinn and J.J. Lister. In several species (notably _Patellina_) plastogamy, the union of the cytoplasmic bodies without nuclear fusion, has been noted, as a prelude to the resolution of the conjoined protoplasm into uninucleate amoebulae. _Calcituba_, a porcellanous type, which after forming the embryonic chamber with its deflected pylome grows into branching stems, may fall apart into sections, or the protoplasm may escape and break up into small amoebulae. Of the reproduction of the simplest forms we know little. In _Mikrogromia_ the cell undergoes fission within the test, and on its completion the daughter-cells may emerge as biflagellate zoospores. The sandy shells are a very interesting series. In _Astrorhiza_ the sand grains are loosely agglutinated, without mineral cement; they leave numerous pores for the exit of the protoplasm, and there are no true pylomes. In other forms the union of the grains by a calcareous or ferruginous cement necessitates the existence of distinct pylomes. Many of the species reproduce the varieties of form found in calcareous tests; some are finely perforated, others not. Many of the larger ones have their walls thickened internally and traversed by complex passages; this structure is called _labyrinthic_ (fig. 19, g, h). The shell of _Endothyra_, a form only known to us by its abundance in Carboniferous and Triassic strata, is largely composed of calcite and is sometimes perforated. [Illustration: FIG. 15.--_Cycloclypeus_.] It is noteworthy that though of similar habitat each species selects its own size or sort of sand, some utilizing the siliceous spicules of sponges. Despite the roughness of the materials, they are often so laid as to yield a perfectly smooth inner wall; and sometimes the outer wall may be as simple. As we can find no record of a deflected stylopyle to the primitive chamber of the polythalamous Arenacea, it is safe to conclude that they have no close alliance with the Porcellanea. _Classification._