70 HISTORICAL PALAONTOLOGY. 
The concentrically-laminated mass of £ozodz is composed 
of numerous calcareous layers, representing the original skele- 
ton of the organism (fig. 23, 4). ‘These calcareous layers serve 
to separate and de- 
fine a series of cham- 
bers arranged in suc- 
cessive tiers, one 
above the other (fig. 
23, A, 3B @ietae 
they are perforated 
not only by passages 
(fig. 23, <2) vcwmien 
Hy  / py serve to place suc- 
ae i cessive tiers of cham- 
bers in communica- 
yrrurys yoru “ tion, but also by a 
YM ~ : 
a a system of delicate 
Fig. 23.—Diagram of a portion of Zozodz cut-verti- branching canals (fig. 
cally. A, B, C, Three tiers of chambers communicating 
with one another by slightly constricted apertures: @ a, 23) @). Moreover, 
The true shell-wall, perforated by numerous delicate the central and prin- 
tubes; 4 4, The main caleareous skeleton (‘‘intermedi- : : 
ate skeleton”); c, Passage of communication (“‘stolon- cipal portion of each 
passage fom one eof chambers another; Rami” calcareous layer, with 
penter.) the ramified canal- 
system just spoken 
of, is bounded both above and below by a thin lamina which has 
a structure of its own, and which may be regarded as the proper 
shell-wall (fig. 23,a@a@). This proper wall forms the actual lin- 
ing of the chambers, as well as the outer surface of the whole 
mass; and it is perforated with numerous fine vertical tubes 
(fig. 24, a a), opening into the chambers and on to the sur- 
face by corresponding fine pores. From the resemblance of 
this tubulated layer to similar structures in the shell of the 
Nummulite, it is often spoken of as the ‘“‘ Nummuline layer.” 
The chambers are sometimes piled up one above the other in 
an irregular manner; but they are more commonly arranged 
in regular tiers, the separate chambers being marked off from 
one another by projections of the wall in the form of parti- 
tions, which are so far imperfect as to allow of a free communi- 
cation between contiguous chambers. In the original condi- 
tion of the organism, all these chambers, of course, must have 
been filled with living matter; but they are found in the present 
state of the fossil to be generally filled with some silicate, such 
as serpentine, which not only fills the actual chambers, but has 
also penetrated the minute tubes of the proper wall and the 
branching canals of the intermediate skeleton. In some cases 
