Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/391

F011AMINIFERA 377 FIG. S.Peneropli Dcndrilina to bo assigned to this type. Now, although such marked differences between Molliiscan shells wculd undoubtedly iustify ijr.ncric differentiation, yet ainong Foraminifera they prove to be not even of S2>ccific value ; for when a suiliciently extensive series of this type is ^yZ^ ^s^sfs^ compared, its extreme forms graduate insen sibly one into the other. In proportion apparent ly to the warmth of its habitat (the gradation being well seen in the Red Sea examples) the shell becomes less flat tened and the septal plane broader, and the single row of pores be comes a double or even a triple scries, the pores of different series being sometimes partially conllr.ent. With a further increase in the turgidity of the spire, and with a progressive widening of the septal plane at the expense of its length, the. pores lose their linear arrangement, and run together in groups, so as to form several irregularly fissured openings ; and it is by the coalescence of these that the single dendritic aperture is formed in those ex amples whose general shape most nearly approaches that of Nautilus. This transition often shows itself in the successive stages of growth of one and the same shell. When we consider the physiological import of this variation, we find that it has no rela tion to any essential differences in the conformation of the animal, all that it means being that the sarcodic pscudopodia, which issue separately in the one case, issue collectively in the other. It is in teresting to remark, further, that in each of these types the spire often straightens itself out, so as to give to the shell the shape of a crazier (these "spiroline" forms being peculiarly common in the Keel Sea), and that throughout the entire series the surface- marking is singularly constant. From this simply-chambered type we pass on to that in which each chamber is more or less completely divided by transverse par titions (or secondary septa) into a series of "chamberlets," giving to the shell what is designated the " labyrinthic " structure. The first stage of this division, shown in figs. 11, 12, consists in a suc cession of contractions by which the sarcodic segment that occupies the cavity of each chamber acquires a moniliform or bead-like shape, the sub-segments into which it is partially subdivided being connected by a continuous baud or "stolon." And even where the separation of the chamberlets is most complete, there is still an aperture in each secondary septum, by which the lateral continuity of the sub-segments is maintained. The relation of the labyrinthic Orbiculina (6 and 8 in fig. 1), common in tropical seas, to the simple " peneropline " type, is well seen in the early stage (7 in fig. 1) of the former, which is obviously a Pcncroplis with subdivided chambers. J .ut as age advances, a remarkable change takes place. The later turns of the spire very commonly grow completely over the earlier, so as to make, the umbilicus protuberant, as shown at 8 in fig. 1 ; and their thickness progressively increases by the vertical elongation of the sarcodic " blocks " that occupy their chamberlets into "columns," which communicate with the exterior and with each other through several ranges of marginal pores. In the very large and highly developed forms of this type (attaining a diameter of 7 or 8 lines), which make up the Tertiary Limestone of the Malabar coast, each surface of the spire has a thin layer of chamberlets divided off from the intermediate columnar portion, as in the " complex" type of Orbitolitcs to be presently described. Now if the axis round which this Orbictiline spire revolves were to undergo such an elongation (by a multiplication of its sub- Kegments in that direction) as to equal the diameter of the spire, FIG. 10. Alveolina: a, a, septal plane, showing multiple ranges of pores. the form would be changed to the globose, with a cresce.ntic septal plane perforated by numerous separate pores ; and a yet further increase in the comparative length of the axis would evolve the " fusiform " type characteristic of Alvcolina (fig. 10), with an elon gated and straightened septal plane showing several ranges of sepa rate pores. The largest specimens of this type at present known seldom exceed 4 inch in length ; and are greatly surpassed by those massive forms of the early Tertiaries whose accumulation in particular localities produced what is now known as the Alveolile Limestone. Sometimes, on the other hand, even at an early age, the growing margin of Orbiculina extends itself laterally at both extremities, so that the two extensions meet on the opposite side of the original spire, which is thus completely enclosed by it. Its growth is thence- FIG. 11. Shell of simple type of Orlitolites, showing primordial chamber n, and circumambient chamber 6, surrounded by successive rings of chamberlets con nected by circular galleries which open at the margin by pores. forth cyclical instead of spiral, concentric rings (each of them answering to one of the chambers of Peneroplis) being successively added, as shown at 6 in fig. 1, and the chamberlets into which they are subdivided opening by separate pores, usually arranged in several series, around the whole margin of the disk. Now, this is the plan of growth characteristic of Orbitolitcs, a type first known as fossil (abounding in the Calcairc grossicr and other calcareous Eocene strata), but of which living forms have been since found abundantly in tropical and sub-tropical seas, frequently attaining a diameter of 8-10ths of an inch, and a thickness approaching l-6th. While mar ginal portions of the disks of the cyclical Orbiculina could not 1m distinguished from those of certain forms of Orbitolitcs, there is such a difference in their origins and early histories as necessitates their generic separation. For in the typical Orbitolite the plan of growth is cyclical from the first, successive rings being formed around a " nucleus," which is never overlaid by later growths ; and this nucleus consists of a "central" segment (a, figs. 11 and 12), which is more or less completely surrounded by a "circumambient seg ment" (b), that gives off numerous radiating stolon-processes to form the first ring of sub-segments. It is a fact of no small morpho logical interest, however, to find among the varietal forms of this type a distinct reversion to the spiral origin, the most complete example of it being presented in the deep-sea 0. tenuissimus, whose " nucleus " resembles a Cornuspira of three or four turns, of which the last turn rapidly opens itself out, showing a primary division into chambers and a secondary into chamberlets, thus distinctly connecting with the ordinary Milioline type the form whose plan of FIG. 12. Animal of simple type of OrWolitei, showing primordial segment a, and circumambient segment 6, surrounded by annul! of sub-segments con- ected by radial and circular stolon-procosscs. growth appears at first sight most fundamentally different. This plan is most readily apprehended by examining such small "simple" forms of this type (figs. 11, 12) as occur in the .&gean and Red Seas, in which the successive rings continue to bo formed on the pattern of the first. Each chamberlet of the shell (fig. 11) is con- IX. 48