GENUS ORBITOLITES; — GENERAL PLAN; COMPLEX TYPE. 
205 
apparent anomaly, which is simply as follows. Each columnar cell really communi- 
cates with the two alternating columnar cells in the next interior zone; but by two 
distinct passages, instead of by the divarication of one ; and these passages are not 
upon the same plane, but those of different planes turn alternately to one side and to 
the other. This is well seen in the two tangential sections represented in Plate VI. 
figs. 8 & 9; of which 8 shows the back or central side of four contiguous columnar 
cells ad, hV , cc', dd', of the same zone, each of them perforated by a series of aper- 
tures, in which some degree of alternation is perceptible; whilst 9 shows the front 
or peripheral side of four other columnar cells, in which it is seen that, by the sinuo- 
sity of the partition, the apertures of any vertical row, even when in a line with each 
other, open alternately into the cells on the right and on the left of the septum ; so 
that {e.g.) the passages extending backwards from the row of apertures in the 
columnar cell hV, fig. 8, will debouch alternately in cells ad and hV, fig. 9, of the 
zone within. The same will of course be true of the pores which open on the margin, 
these being nothing else than the orifices of the inter-zonular passages just described, 
which, when another annulus is added, lead into its cells. This idea of the alterna- 
ting direction of the inter-zonular passages, seemed to furnish the solution of the 
appearances presented in Plate VI. fig. 2 ; for, as the disks are seldom perfectly flat, 
the section which traverses, at one part of the disk, the set of passages running in 
one direction, will traverse the other set of passages, where, by the flexure of the disk, 
the plane of seetion is slightly altered in regard to it. All doubt, however, as to the 
validity of this explanation, was removed by the examination of the animal substance 
filling the vertical columns ; for, as is shown in Plate IV. fig. 4, each column of sarcode 
in one zone {cc) does communicate with the two columns alternating with it in the 
next zone (cV) by two rows of peduncular stolons ; and the peduncles which pass 
from each pair of contiguous columns, to the single column of the next zone, incline 
towards one another, so as to enter it nearly in the same vertical line, though in dif- 
ferent horizontal planes. 
31. That which has been already stated in regard to the partial deficiency of the 
inner wall in each of the concentric zones of the simple type (5[ 21.), holds good also 
in regard to the septa which divide the successive zones of the intermediate stratum 
in this more complex type; for the walls of the columnar cells close-in around them 
very imperfectly on their inner or central side, leaving large irregular vertical fissures 
(Plate VIII. fig. 1) which are applied to the vertical rows of orifices (Plate VIII. 
fig. 2) on the outer margin of the included zone. 
32. The thickness of this intermediate stratum, and the number of vertical segments 
of which it consists, are found to vary considerably in different parts of the same 
disk ; being usually least near the nucleus, and gradually augmenting in successive 
zones as their distance from the centre increases (Plate VI. fig. 7) ; or ceasing to 
augment at a certain point, so that the outer part of the disk is flat; or even dimi- 
nishing again, so that the disk thins away towards its margin. It is specially worthy 
2 E 
MDCCCLVI. 
