GENUS ORBITOLITES: — GENERAL PLAN OF ORGANIZATION. 
195 
Stance of the peripheral segments of the Orbitolite-body can only be brought together 
towards the centre, through being completely unattached to the walls of the cavities 
which it occupies, and through having a form so alterable, as to be capable of being- 
drawn in threads through the narrow connecting passages, and of then coalescing 
together again so perfectly, that the masses they form do not present the least trace 
of having been tiuis spun out. There is no known kind of animal texture, except 
sarcode, that is susceptible of this kind of alteration ; and the evidence of it which I 
have adduced seems to me extremely valuable, not only as establishing the general 
nature of the animal body of the Orbitolite, but also as fully justifying the assump- 
tion, that, in the living state, the sarcode is projected in pseudopodia through the 
marginal apertures, and that alimentary particles are introduced by their instrumen- 
tality, as in other Foraminifera. 
13. Turning from the animal body to the calcareous disks which enclose it, we find 
that, whether large or small, these are almost invariably circular, or nearly so; that 
they are usually nearly flat, any difference in thickness being generally in favour of 
the marginal portion; and that if, as sometimes happens, there is a slight central 
projection, this is formed by the nucleus alone. By these characters we may distin- 
guish OrbitoUtes from Ovbiculina\ for although the discoidal forms of the latter so 
strongly resemble Orbitolites, that by the structure and arrangement of their mar- 
ginal portion they could not be distinguished, yet they may always be discriminated 
by the knobby protuberance of their centre, which is occasioned by the mutual 
investment of the earlier whorls of the spiral in which they commence. The same 
entire absence, or very small size, of the central elevation, together with the uni- 
formity or even slight increase of thickness towards the circumference, also helps 
us to separate Orbitolites from Orbitoides ; the centre of the latter being always 
considerably elevated, and the thickness of its disk ordinarily diminishing gra- 
dually towards its margin Around the ' nucleus’ which occupies the centre of 
the disk (Plate V. figs. 1, 6), are seen an indeterminate number of concentric zones 
of cells (c, c, c), the shape of which differs in different individuals (see Sect. IV.) ; 
these, although completely closed (unless laid open by abrasion), have their form 
* I wish this statement to be understood with reference to the genus Orbitoides, as characterized by the 
structure which I have shown it to possess (Quart. Journ. of Geol. Soc., Feb. 1850), and not to the genus as 
defined by M. d’Orbigny (Cours Elementaire de Paleontologie, tome ii. p. 194), who, notwithstanding that 
he has shown himself to be acquainted with my Memoir (by copying from it a figure of Nummulite), has not 
profited in any degree by my investigations, but has left the generic characters of Orbitolites, Orbitolina and 
Orbitoides in the state in which they might have been, and probably were, before that Memoir was published. 
The true distinction, however, has been fully recognized by M. d’Archiac, who, in his ‘ Description des Ani- 
maux Fossiles du Groupe Nummulitique de ITnde,’ p. 349, has designated as Orbitoides dispansa and Orbitoides 
Fortisi, the bodies which, in the account of them he had previously given in the ‘ Mem. Soc. Geol. de France,’ 
2nd ser., vol. iii., he had designated as Orbitolites; thus correcting the error into which Mr. Carter has 
fallen in his description of the same fossils, through reliance on M. d’Orbigny’s insufficient and indeed erro- 
neous characters of these genera. 
