TWO GIGANTIC TYPES OF ARENACEOUS FORAMINIFERA. 
729 
the ‘ radial tubes ’) which separates it from the lamella that encloses it, may be conve- 
niently considered as one layer ; and it will be found to be entirely on the differences in 
the proportions of these two components, that the diversities of aspect presented by the 
several parts of the sectional plane depend. Thus it will be observed in Plate LXXII., 
and on a larger scale in Plate LXXIII. fig. 1, that in the twelve concentric layers which 
surround the ‘ nucleus,’ the breadth of the interspaces is twice or thrice the thickness 
of the solid lamellae ; but that in certain parts of the outer four of these layers, the 
solid lamellae send irregular ‘ processes ’ across the interspaces, so that the ‘ labyrinthic 
systems’ of successive lamellae are brought into connexion not merely by the ‘ radial 
tubes,’ but by direct continuity of cancellated structure. These layers are surrounded 
by a layer (Z 1 , l 1 ) of labyrinthic structure unbroken by any interspace, which is equal in 
thickness to any two of the layers it encloses. We then come to a second series of 
twelve layers, of greater thickness than the first ; the increase being due to the greater 
development of the ‘labyrinthic system’ of each lamella, whilst the breadth of the in- 
terspaces remains the same ; so that the breadth of the interspaces and that of the solid 
lamellae are now nearly equal. But we here notice that the interspaces, instead of being 
traversed by the irregularly disposed ‘ radial tubes ’ of the inner layers, are crossed at 
pretty regular intervals by ‘ radial processes ’ of labyrinthic substance, which (as will be 
shown hereafter) form an investment to the radial tubes ; and thus the concentric inter- 
spaces that separate the successive solid lamellae from the lamellae internal and external 
to them, are divided into a number of small cavities having little communication with 
each other. This series of layers is separated by a second thick lamella (l 2 , l 2 ) of laby- 
rinthic substance, the breadth of which somewhat exceeds that of the first. And around 
this is a third series of seven concentric layers, which closely resemble those of the 
second, except that the radiating bands of labyrinthic substance are broader, so that 
the interspaces which they cross and divide form in the aggregate but a yet smaller 
proportion of each layer. The thick lamella (l 3 , l 3 ) of labyrinthic substance which 
encloses these, the five ordinary layers by which it is invested, and the yet thicker 
lamella ( l 4 , Z 4 ) of labyrinthic substance which forms the exterior of the sphere, corre- 
spond in all essential particulars with those already described. 
12. Thus we see that in passing from the centre to the circumference of the sphere, 
we meet with a progressive increase in the proportion which the comparatively solid 
‘labyrinthic system’ bears to the ‘ interspace -system’ of the successive layers; so that 
the solidity of the fabric as a whole augments with its increase in size. And this seems 
to be a general character of the type ; although there is found to be no constancy, when 
different individuals are compared, in regard to the number of ordinary layers that inter- 
vene between the peculiarly thick lamellae by which the general regularity of the former 
is interrupted. 
13. We shall now examine, in more detail, such a portion of a group of Concentric 
Layers about halfway between the centre and the periphery of the sphere, as may be 
taken to present the plan of structure characteristic of the fabric generally ; after the 
mdccclxix. 5 e 
