1. Monostega.—Body consisting of a single segment : shell of one chamber.
2. Stichostega.—Body composed of segments disposed in a single line : shell consisting of a linear series of chambers.
3. Helicostega.—Body consisting of a spiral series of segments: shell made up of a number of convolutions.
4. Entomostega.—Body consisting of alternate segments spirally arranged : shell chambers disposed on two alternating axes forming a spiral.
5. Enallostega.—Body composed of alternate segments not forming a spiral : chambers arranged on two or three axes which do not form a spiral.
6. A gathistega.—Body consisting of segments wound round an axis : chambers arranged in a similar manner, each investing half the entire circumference.
A somewhat different arrangement has been adopted by Schultze, who divides the Polytho 1 a mia into three sections, viz.— 1. Helicoidea, including those forms in which the several chambers of the shell are arranged in a convolute series, and answering to the last four orders of d'Or bigny.
2. Rhalxioulea, in which they are placed in a direct line (Stichostcga , d'Orb.) ; and 3. Soroidea, where they are disposed in an irregular manner (Acervulina).
Lagena is a genus of Monostega, or single-chambered Foraminifera, with a flask-shaped shell, sometimes pre senting a beautiful fluted exterior. Entosolenia is like a Lagena, with the tubular neck inverted into the cavity of the shell.
Among the many-chambered Foraminifers the modifica tions of form seem endless. Nodosaria resembles a cylin drical beaded rod : Cristellaria begins by being spiral and afterwards becomes straight : most species are wholly spiral : in some, as Nummulites, the convolutions are on the same plane : in many the spiral turns obliquely round an axis, and gives the shell a trochoid form.
Upwards of six hundred and fifty-seven fossil species, belonging to seventy-three genera, have been described : they commence in the palieozoic age, increase in number and variety with each successive stratum, and attain their maxi mum in the present seas. Most of the fossil genera, and even some of the species, pass through many formations ; indeed, if correctly observed, the existing forms are the oldest known living organisms. Dentalina communis, Orbitolitcs complanatus, Rosalina italica, and Rotalina globulosa, all living species, are said to be found in the chalk ; Rotalina umbi licata ranges to the gault ; and Weblnna rugosa is common to the upper lias, the chalk, and present sea. It has, however, been observed, that fossil Rhizopods, set free by the disinte gration of rocks, are mingled with the recent shells on every beach ; and Mr. M'Andrew has obtained them in this condi
tion from great depths of the mid-channel.
The earliest important form is the Fusulina (fig. 2, s), which forms layers many inches, or even feet, in thickness in the carboniferous limestone of Russia. The recent genera Dentalina and Tertularia are found in the magnesian lime stone ; Nodosaria, CrLstellaria, and Rotalia, in the lias. Fla bellina (fig. 2, 6) is peculiar to the chalk ; Orbitoides (fig. 2, 9) to the chalk and tertiary series ; Ovulites (fig. 2, io) is peculiar to the eocene tertiaries; Operculina, Orbitolites, and Alvcolina appear first in the tertiary, and are still living. The Lit uola (fig. 2, 7) occurs in the chalk and chalk flints, and has been described as a species of " Spirolinite." Many of the cretace ous Foraminifera contain a brown colouring matter, which remains after the shell has been dissolved with weak acid, and has been regarded as the remains of the organic substance which once tilled all the cells.
The " calcaire grossier," which is employed at Paris as a building-stone, contains Foraminifera in such abundance that one may say the capital of France is almost constructed of those minute and complex shells.
But it is in the middle eocene, or " nummuLitic period," that the Rhizopods attained their greatest size, and played their most important part. Wherever limestones or calcare ous sands of this period are met with, these Foraminifers abound, and literally form strata which in the aggregate become mountain masses. These " nummulitic limestones" are found in Southern Europe, in Northern Africa, and in India ; they also occur in Jamaica. The commonest form is the Nummdite (fig. 2, s), which occurs in the building-stone of the Great Pyramid. The Nummulites were evidently sedentary organisms ; and, in the large thin species, one side is moulded to the inequalities of the sea-bed on which it grew.
Polycystinere.—The tertiary marls of Barbadoes afforded to Ehrenberg an extensive series of novel and extraordinary microscopic organisms, composed of silica, but foraminated like the shells of the Rhizopods. The same forms, and others similar to them, have been met with in the deep-sea mud of the Gulf of the Erebus and Terror, and more recently in the mud of the North Atlantic soundings. They are quite dis tinct in form and character from most of the silicious-shielded Diatomaceoe, but some of them resemble the Coscinodiscus. and Actinoeyelus. No less than 282 forms, grouped in 44 provi sional genera, have been described.