GENUS ORBlTOLITES: GENERAL PLAN; SIMPLE TYPE. 199 



the circumambient segment of the nucleus is seen to give off a number of slender 

 prolongations or 'stolons' of sarcode, which radiate from it to a short distance and 

 then enlarge into columnar segments having a circular or ovoidal base, which are 

 united with each other laterally by an annular 'stolon'; and from the portions of this 

 ' stolon' which intervene between the segments of each annulus of sarcode, are given 

 off the radiating 'stolons' that go to originate the next zone, the arrangement of 

 whose parts is precisely similar to that just described. 



18. In this manner, any number of concentric zones may be formed, which are 

 exact repetitions of each other, except that the number of segments in the outer 

 zones is greater than that of which the inner zones are composed. It does not 

 increase, however, in the regular ratio of the respective diameters of the zones ; for 

 the cells of the outer zones, being usually both larger and more widely separated 

 from each other than are those of the inner, are less numerous in proportion ; thus 

 in a specimen before me, there are twenty-eight cells in the innermost row and only 

 forty-nine in the outermost, though the latter is more than twice the diameter of the 

 former. The increase in the number of the segments is accomplished by the occa- 

 sional interpolation of an additional segment, communicating directly with the one 

 immediately interior to it, between the two segments which spring from the annular 

 stolon on either side of the latter, as is shown in Plate IV. figs. 8, 9, a. Hence it is 

 obvious that prolongations of sarcode giving origin to new segments, although ordi- 

 narily put forth rather by the connecting 'stolons' than by the segments themselves, 

 may originate from any part of the annulus. This is shown still more forcibly by the 

 occasional occurrence of irregularities, such as that represented in Plate VI. fig. 6. 



19. The cells of the last-formed zone communicate with the exterior by the very 

 same kind of radial passages, as in other instances communicate with the next zone ; 

 and the external orifices of these form the pores which present themselves at the 

 margin of the disk (Plate V. fig. 1). Thus it is seen, on the one hand, how it hap- 

 pens that these pores are intermediate between the cells, instead of opening directly 

 into them ; and on the other, how each pore leads, by the divarication of its passage, 

 into two cells, one on either side of it. When a new zone is formed, each pore opens 

 into one of its cells ; and this zone in its turn communicates with the exterior, 

 through a new set of pores at its own margin. Each pore is often surrounded by a 

 rather prominent annulus of shell (Plate V. fig. \,d); and it is obvious that when 

 the section passes through this, it will be indicated by a little ' beak' on either side of 

 the entrance to the passage ; such ' beaks' (which are of course repeated through the 

 entire disk) are shown in their ordinary aspect in Plate IV. fig. 9, but they are fre- 

 quently more prominent, as is shown in Plate VI. fig. 1. 



20. In all cases in which the growth of the disk takes place with normal regula- 

 rity, a complete circular zone is added at once. Exceptions to this regularity are 

 rare, and they can be generally traced with probability to some accidental interrup- 

 tion. It can scarcely be doubted, I think, that when a new ring of cells is about to 



2 D 2 



