440 FORAMINIFERA, POLYCYSTINA, AND SPONGES. 



deposits being added, sometimes in the shape of bosses, spines, 

 or other outgrowths, that a soft substance, capable of originating 

 such new formation, must occasionally spread itself over the 

 whole external surface of the previous segments. And it is not 

 difficult to understand how this may come to pass, when it is 

 borne in mind that the gelatinous sarcode, however fine may be 

 the threads into which it divides itself, may readily form a con- 

 tinuous layer by the coalescence of those threads. The group of 

 Helicostegues is subdivided by M. D'Orbigny into two families, 

 the Nautiloidce, and the Turbinoidce ; the first and most important 

 consisting of those in which the successive whorls all lie in the 

 same plane, so that the shell is " equilateral" (like that of a Nau- 

 tilus), as is the case with Nummulites and their allies (such as 

 Nonionina, Assilina, Operculina, &c., between which and Num- 

 mulites the differences are but very slight); whilst the second 

 contains those in which the spiral passes obliquely round an axis, 

 so that the shell becomes "inequilateral" (like that of a snail or 

 periwinkle), as is the case with Rotalia (Fig. 335), Faujasina 

 (Fig. 209) and Rosalina (Fig. 205). 



287. Putting aside less important variations in the plan of 

 gemmation, we have now to notice one that seems essentially dis- 

 tinct from the preceding ; that, namely, in which the new seg- 

 ments are added in concentric rings, each surrounding its pre- 

 decessors, so as to form flattened disks. As an example of this curi- 

 ous type of Foraminiferous structure, the Orlitolite may be cited ; 

 which, long known as a very abundant fossil in the early ter- 

 tiaries of the Paris basin, has lately proved to be scarcely less 

 abundant in certain parts of the existing ocean. The largest 

 specimens of it, sometimes attaining the size of a sixpence, have 

 hitherto been obtained only from the coast of New Holland and 

 various parts of the Polynesian Archipelago ; but disks of com- 

 paratively minute size (from the diameter of an ordinary pin's 

 head, to that of a small pea) and of simpler organization, are to 

 be found in almost all Foraminiferous sands and dredgings from 

 the shores of the warmer regions of the globe, being especially 

 abundant in those of some of the Philippine Islands, of the Red 

 Sea, of the Mediterranean, and especially of the ^Egean. When 

 such disks are subjected to Microscopic examination, they are 

 found (if uninjured by abrasion) to present the structure repre- 

 sented in Fig. 206 ; where we see on the surface (by incident 

 light) a number of rounded elevations, arranged in concentric 

 circles around a sort of nucleus (which has been laid open in the 

 figure to show its internal structure) ; whilst at the margin we 

 observe a row of rounded projections, with a single aperture or 

 pore in each of the intervening depressions. In very thin disks, 

 the structure is often brought into view, by mounting them in 

 Canada balsam, and transmitting light through them, sufficiently 

 well to render any other mode of preparation unnecessary ; but 

 in those which are too opaque to be thus seen through, it is suffi- 



