Although the character and structure assumed by the semi-fluid bodies of the known Foraminifera have been determined in most cases with admirable precision, it is still thought advisable to arrange them according to the substance of the shell: consequently they form three natural orders; namely, the Porcellanous or imperforate, which have calcareous shells often so polished and shining that they resemble porcelain; secondly, the Arenaceous Foraminifera, consisting of animals which secrete a kind of cement from their surfaces, and cover themselves with calcareous or siliceous sand-grains; and lastly, the Vitreous and Perforated order, which is the most numerous and highly organized of the whole class, has siliceous shells transparent as glass, but acquires more or less of an opaque aspect in consequence of minute straight tubes which perforate the substance of the shell perpendicularly to its surface, and consequently interfere with the transmission of light.
Order of Porcellanous Foraminifera.
The Miliolidæ constitute the porcellanous order, which consists of twelve genera and many species, varying from a mere scale to such as have chambered shells of complicated structure.
The genus Miliola has minute white shells resembling millet seeds, often so brilliantly polished that they are perfectly characteristic of the porcelain family to which they belong. No Foraminifera are better suited to give an idea of the intimate connection between the shell and its inhabitant than the Miliola, the fundamental type of this genus. The shell is a spiral (I, [fig. 97]), which is made up of a series of half turns arranged symmetrically on its two sides. Each half turn is longer and of greater area than that on the opposite side, so that each turn of the spire has a tendency to extend itself in some degree over the preceding one, which gives a concave instead of a convex border to the inner wall of the chamber. The sarcode body of the Miliola consists of long segments which fill the chambers, connected by threads of sarcode passing through the tubular constrictions of the shell. As the animal grows, its pseudopodia extend alternately now from one end, and now from the other extremity of the spiral, and by them it fixes itself to seaweeds, zoophytes, and other bodies, for these Foraminifera never float or swim freely in the water. The genus Miliola is more extensively diffused than almost any other group of Foraminifera; they are most abundant between the shore and a depth of 150 fathoms, and are occasionally brought up from great depths. Beds of miliolite limestone show to what an extent the Miliola abounded in the seas of the Eocene period; but the type is traced back to the Lias.
The genus Peneroplis is distinguished by a highly polished opaque white shell; its typical form is an extremely flat spire of two turns and a half opening rapidly and widely in the last half whorl. It is strongly marked by depressed bands which indicate the septa or shelly partitions between the chambers in the interior. The polished surface of the shell is striated between and transversely to the bands by parallel platted-looking folds 1⁄1400 of an inch apart. But the peculiarity of this shell and its congeners is, that the partitions between the chambers in its interior are perforated by numerous isolated and generally circular pores which in this compressed type are in a single linear row. Their number depends upon the length of the partition between the chambers, which increases with the age of the animal and size of the shell. There is but one pore in each of the consecutive partitions from the globular centre to the fourth chamber. From the fourth to the seventh chamber the communication is by two pores; after this the number is gradually increased to three, four, six, &c., up to forty-eight, so that the last segment may send out forty-eight pseudopodia from the mouth of the shell. In its early youth one pseudopodium appears to have been sufficient to find food for the animal, but as the shell increased in size and the segments in number, a greater supply of food was requisite and a greater number of pseudopodia were necessary to fish for it. Moreover when an addition to the shell is required the pseudopodia coalesce at their base and form a continuous segment upon which the new portion of the shell is moulded.
In varieties of the Peneroplis where the spire is less compressed there are sometimes two rows of pores in the partitions between the chambers. The Dendritine variety deviates most from that described. It is characterised by a single large aperture in each partition which sends out ramifications from its edges. The form of these openings depends upon that of the spire; when compressed the aperture is linear and less branched at its edges; but in shells which have a very turgid spire it is sometimes broader than it is long, and much branched; but these extremes are connected by a variety of forms. The shells of this variety of the Peneroplis are strongly marked by the depressed bands and striæ, as in the Dendritina elegans (F, [fig. 97]). The segments of the animal inhabiting these shells must be more intimately connected than in most of the other Foraminifera; and the pseudopodia sent through these large apertures out of the mouth of the shell must be comparatively quite a mass of sarcode. The Dendritinæ are inhabitants of shallow water and tropical seas, while the other members of the genus Peneroplis abound in the Red Sea and the seas of other warm latitudes, especially in the zone of the great laminarian fuci. They do not appear in a fossil state prior to the beginning of the Tertiary period.
The last whorls of some of the compressed spiral Foraminifera of the Porcellanous order so nearly encompass all their predecessors, that the transition from a flat spiral to the Orbitolite with its flat disk of concentric rings is not so abrupt as might at first appear. The gradual change may be distinctly traced in the species of the genus Orbiculina. The exteriors of the shells of the genus Orbitolites have less of the opaque whiteness than many others of its family. In its simplest form it is a disk about the 1⁄500 of an inch in diameter, consisting of a central nucleus surrounded by from ten to fifteen concentric circular rings. The surface is usually plane, though sometimes it is concave on both surfaces in consequence of the rings increasing in thickness towards the circumference. The rings or zones are distinctly marked by furrows on the exterior of the shell, and each of these zones is divided by transverse furrows into ovate elevations with their greatest diameter transverse to the radius of the disk, so that the surface presents a number of ovate elevations arranged in consecutive circles round the central nucleus. The margin of the disk exhibits a series of convexities with depressions between them; in each of these depressions there is a circular pore surrounded by a ring of shell: these pores are the only means the animal possesses of communicating with the water in which it lives.
Fig. 98. Simple disc of Orbitolites complanatus.
