GENUS ORB1TOLITES: 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 thus 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 Orbitolites from Orbiculina ; 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 

 denned 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'AHCHIAC, who, in his ' Description des Ani- 

 maux Fossiles du Groupe Nummulitique de 1'Inde,' p. 349, has designated as Orbitoides dispansa and Orbitoides 

 Fortisi, the bodies which, in the account of them he had previously given in the ' Me'm. 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'ORBIGNT'S insufficient and indeed erro- 

 neous characters of these genera. 



