SHELLS OF FORAMINIFEKA. 9 



the enlargement or non-enlargement of the successively formed cham- 

 bers ; for sometimes it opens out very rapidly, every whorl being con- 

 siderably broader than that which it surrounds, in consequence of the 

 great excess of the size of each segment over that of its predecessor 

 (fig. 2, 1) ; but more commonly there is little difference between the 

 successive segments after the spiral has made two or three turns. In 

 many genera the new segments are added in concentric rings, each 

 surrounding its predecessors, so as to form flattened disks varying in 

 size from that of a pin's head to that of a sixpence. When such disks 

 are subjected to microscopic examination, they are seen to be composed 

 of concentric circles of cells (fig. 3) which communicate with each 



Fig. 3. 



Structure of the calcareous disk of an Orbitolite : a, the central cell; 5, circumambient cell; 

 c, c, concentric zones; d, d, annular passages of the outermost zone. The same parts are shown 

 by a vertical section passing in a radial direction at e, e ; and a,tf,f, following the course of one 

 of the zones. 



other by means of lateral passages, and which in the living state are 

 each of them filled by the sarcode whereof the living portion of the 

 animal consists. In this case there can be no reasonable doubt that the 

 radial extensions of the outermost zone issue forth as pseudopodia from 

 the marginal pores, and that they search for and draw in alimentary 

 materials in the same manner as do those of other Foraminifera. 



(21.) Where the growth of the disk takes place with normal regu- 

 larity, it is probable that a complete circular zone is added at once. When 

 the sarcode body has increased beyond the capacity of its enveloping disk, 

 it may be presumed that its pseudopodial extensions proceeding from 

 the marginal pores coalesce, so as to form a complete annulus of sarcode 

 round the margin of the outermost zone ; and probably it is by a deposit 

 of calcareous matter in the surface portion of this annulus that the new 

 zone of shelly substance is formed, which constitutes the walls of the 



