Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/390

 37(5 FOBAMINIFERA found at much greater depths, ranging downward to more than three miles. r&amp;gt;ut this more developed form is characterized by the exten sion of the principal cavity of each chamber into passages excavated in its thick external wall, each passage being surrounded by a very FIG. G. iiutiloid Lituola. a, exterior; b, chambered interior; &amp;lt;, portion of labyrinthic chamber- wall, showing component sand-grains. regular arrangement of sand-grains, as seen at c. This labyrin thic &quot; structure is of great interest, from its relation on the one hand to the still higher development of the like structure in the large fossil Lituola of the Chalk, whose crozier-like &quot;test &quot; has its principal chambers subdivided into &quot; chamberlets,&quot; and on the other hand to that of two gigantic fossil Arcnacea, one of the Cre taceous and the other of early Tertiary age. Geologists who have worked over the Greensand of Cambridgeshire have long been familiar with solid spherical bodies, varying in size from that of a pistol-bullet to that of a small cricket-ball, which there present themselves not unfrequently, and have been regarded by some as mere mineral concretions, whilst others supposed them to be fossilized Sponges. They prove, however, to be &quot;tests&quot; of Arenaceous Foraminifera, which are distinguished from all other known foraminiferal types by their concentrically-spherical plan of growth (fig. 7), the successive spheres being formed around a Fio. 7. General view of the internal structure of Parlceria. In the horizontal section, I 1, ft, I 3 , I*, mark the four thick layers ; in the vertical sections, A marks the internal surface of a layer separated by concentric fracture ; B, the appear ance presented by a similar fracture passing through the radiating processes; c, the result of a tangential section passing through the cancellated substance of a lamella; n, the appearance presented by the external surface of a lamella separated by a concentric fracture which has passed through the radial pro cesses ; E, aspect of section taken in a radial direction so as to cross the solid lamella and their intervening spaces ; c 1, c 2 , c 3 , c 4 , successive chambers of nucleus. _. . &quot; nucleus,&quot; which consists of a succession of chambers arranged sometimes in a straight line and sometimes in a spiral, the last of them opening out and extending itself over its predecessors. Each .sphere contains a set of principal chambers, with labyrinthic extensions of them, corresponding in all essential particulars with those of the nautiloid Lituola just described ; so that there is no thing except its large size (which simply results from continuity of increase and spherically-concentric plan of growth) to differentiate Parkeria from ordinary Foraminifera. It is not a little interesting to iiud the very same plan of construction carried out under a dif ferent form in another gigantic fossil Arenacean, probably of the early Tertiary period, termed Loftusia ; which, agreeing with Parkcria all essential particulars, differs from it in plan of growth, Loftusia being formed, like Alveolina (fig. 10), by the rolling of a spiral round an elongated axis. The discovery of these two singular types greatly enlarges our con ceptions of the Arenaceous group, and shows how little justification there is for any attempt to lay down a priori restrictions as to the supposed &quot; possibilities &quot; of Foraminiferal organization. _, II. PORCELLANEA. In the sketch now to be given of the &quot; porcellanous &quot; series, it will be the writer s object to show the closeness of the relation between its simplest and its most complex and diversified forms (which he regards as all constituting but a single family, the Miliolidct), and to indicate the chief of those gradafcional transitions by which a continuous affinity is established between them all. Passing by the proteiform Nubecularia^ the tent-like incrusting shells of which rudely foreshadow almost every plan of growth to be met with among higher Foraminifera, we commence with the simple Cornuspira (1 in fig. 1), whose spirally-coiled undivided tube, the exact isomorph of the helical arenaceous Trocliammina, attaches itself to the Zoophytes and Sea- weeds of the British coasts. In full-grown specimens, the later turns of the spire generally flatten themselves out, like those of Peneroplis (5 in fig. 1), so that the form of the mouth is changed from a circle to a long narrow slit. From this simple undivided spire we may pass along two divergent lines, one conducting us to the miliolinc and the other to the orbiciiline type, and the latter again leading us to the orbitoline, in which the spiral plan of growth gives place in the very earliest stage to the cyclical. The shells of the MiliolcK (so named from their resem blance to millet-seeds) are at present found in the shore sands of al most every sea ; and they accumulated in certain localities during the early Tertiary period to such an extent as to form massive beds of limestone. Their typical forms (3 and 4 in fig. 1) are composed of a succession of elongated chambers, usually arranged bi-serially on the opposite sides of a straight axis (3 in fig. 1 ), at one of the ends of which each chamber opens forwards into that which follows, whilst at the opposite end it opens backwards into that which preceded, the aperture in either case being partially closed by a &quot; tongue &quot; or &quot; valve &quot; (which may be considered an imperfect septum), as shown at 4, fig. 1. Indications of such a division may be sometimes noticed even in Cornuspira, but it shows itself fully in Spiroloculina, in which the spire has the form of an ellipse, at the two ends of whose major axis the new chambers commence alternately. The spiral plan of growth here remains obvious throughout ; but in the typical Milio- lines (3 and 4 in fig. 1) it is more or less obscured by the extension of the later chambers over the earlier, giving rise to the varied forms which have been distinguished under the generic .names ofHiloculina, Triloculina, and Quinqueloculiiut, according to the number of cham bers visible externally. The extreme inconstancy, however, which is found to mark this distinction, when large numbers of specimens are compared, entirely destroys its value as a differential character. Milioline shells, though frequently quite smooth, often show some kind of &quot;sculpture,&quot; their surface being sometimes more or less deeply &quot;pitted, &quot;sometimes regularly honeycombed, and sometimes longitudinally ribbed ; and these diversities have been largely used for the characterization of species. Here again, however, the exam ination of a sufficiently large collection shows the futility of any such differentiation, since every gradation is found between the smooth and the most sculptured forms, while it is not at all uncom mon for one and the same shell to be smooth on some parts of its surface, and deeply pitted, honey-combed, or ribbed on others making it obvious that such differences have no more than a varietal significance. There, is not, in fact, any group in the whole Animal Kingdom of which the study is more instructive than that of the Miliolincs, in regard to the extent of variation which is com patible with conformity to a fundamental type. The culminating form of the Milioline type may be considered to show itself in Fabularia, a fossil of the Paris Tertiaries not known to exist at present, in which the principal chambers are subdivided into &quot;chamberlets,&quot; as in .Orbiculina and Orbitolitcs,a.ud the aperture is cribriform instead of single, the chamberlets opening externally by separate pores. The undivided helical tube of Cornuspira, again, graduates through Hauerina and the spiral forms of Vcrtcbralina into the typical Pcnc- ropli-s (fig. 8), a beautiful little shell that abounds in the shallow watersof many parts of the Mediterranean, and is remarkable alike for its glistening porcellanous surface, for the constancy of the ridge and furrow sculpturing which crosses the intervals between the septa, and for the llattening-out of its spire, and consequent narrowing and elongation of its septal plane (5a in fig. 1), which is perforated by a single linear series of pores. In tropical seas, however, this typo shows itself under a very different aspect (fig. 9). The spire, instead of being flattened out, is turgid ; and the septal plane, in stead of being perforated by a linear series of pores, has a single large aperture, the dendritic shape of which has caused the name