732 DE. W. B. CAEPENTEE AND ME. H. B. BEADY ON 
system which surrounds its base ; and still better in the transparent section represented 
in Plate LXXYI. fig. 1, where two of the interspaces are seen to be traversed by ‘radial 
tubes’ t 1 , f, which do not form part of the ordinary ‘radial processes.’ A careful exa- 
mination of the entire section of which only a small part is here figured, has fully satis- 
fied me of the universality of this communication ; notwithstanding that (as at the upper 
part of the tube t , Plate LXXIII. fig. 4) it often appears to be interrupted, — the fracture 
or section not having happened to lay open the apertures or passages of connexion. And 
further, I have been enabled to satisfy myself, by the use of adequate magnifying power, 
that notwithstanding the smooth shelly aspect which the walls of the ‘radial tubes’ often 
present, they are in reality built up, like the lamellae between which they pass, of aggre- 
gated sand-grains, — a fact which may be regarded as justifying the like interpretation of 
the appearance presented by the substance of the Nucleus (§ 10). 
16. When a sufficiently high magnifying-power is applied to transparent sections 
thin enough to bear it, so as to bring into view the forms of the individual sand- 
grains and the mode of their aggregation, a curious diversity is observable as to both 
these particulars between different individuals. In the two non-infiltrated specimens 
I have thus examined, the sand-grains are angular, and are fitted together with marvel- 
lous exactness, as shown in Plate LXXV. fig. 2 ; in which we see, moreover, that each 
of the partitions, p, p, which separate the chamberlets c, c, is formed of at least three 
layers of apposed sand-grains. In the specimens which have undergone calcareous in- 
filtration, the forms of the individual sand-grains cannot be clearly made out ; but in 
the thickness and solidity of the partitions between their chamberlets, they agree with 
the preceding. In the specimen which has undergone siliceous infiltration, however, — 
of which the general structure as displayed in section has been already demonstrated 
(Plate LXXVI. fig. 1), — the application of a higher magnifying-power shows that the 
individual sand-grains have a somewhat rounded form (fig. 2), and are more loosely 
fitted together ; and further, that the partitions between the chamberlets are formed of 
only a single layer of sand-grains. Hence the cement furnished by the animal must 
have probably borne a larger proportion to the sand-grains obtained from without, than 
it did in the spheres of which the component sand-grains are so closely fitted together 
that there seems no room for any uniting medium ; and the arenaceous structure of this 
individual must have been far less solid and compact than that of any of the other 
specimens yet examined. The difference in the form of the sand-grains must have been 
pretty certainly due to difference of locality, this specimen not having been obtained 
with the others, but forming part of a distinct collection. Whether the difference in 
general solidity had any relation to the nature of the material employed, can of course 
be only determined by the examination of other specimens whose component sand-grains 
exhibit the same character ; but it seems a possible supposition that as the rounded form 
of the sand-grains must have required more cement to unite them, a limitation in the 
quantity of this cement capable of being furnished by the animal might have prevented 
it from aggregating the number of sand-grains which are found in such close apposition 
