GENUS CYCLOCLYPEUS : — ORGANIZATION. 
557 
99. But although the existence of only a single layer of chambers is obviously the 
rule in this species, yet exceptions to it are not unfrequent ; a subdivision of the entire 
stratum into two or three presenting itself when its thickness is above the average, 
as is shown in Plate XXXI. fig. 8, a. Occasionally one or two chambers only are 
thus subdivided, as shown in figs. 4, 5. The cavity of each chamber is surrounded by a 
proper wall of its own, quite distinct from that of the chambers which it adjoins ; and 
hence the septum by which each chamber is divided from the adjacent one on either 
side, is formed of at least two lamellae (Plate XXIX. fig. 12). These come into close 
contact with each other at the junction of the vertical septum with the horizontal 
roof and floor of the chamber, as shown in Plate XXXI. figs. 2, 4, 5 ; but elsewhere 
they diverge from one another, leaving an interseptal space, which is partly filled-up 
by an interposed lamina of shell-substance, but is partly occupied by the interseptal 
canals to be presently described. A thicker space of the same kind is in like man- 
ner left between the proper walls of the chambers forming one annulus, and those of 
the chambers forming the annuli internal and external to it : this space is almost 
entirely filled-up by a shelly deposit, the interseptal canals which pass between the 
successive annuli being less numerous than those which run between the chambers 
of the same annulus 105). 
100. As in OrhitoUtes, the chambers of each annulus usually alternate in position 
with those of the annuli internal and external to it. But this is by no means con- 
stantly the case ; since additional chambers are ‘ interpolated’ here and there, so as to 
increase the number according to the augmented diameter of the annulus; and such 
an interpolation disturbs the regular arrangement of the neighbouring chambers. 
101. The adjacent chambers of the same annulus have not, so far as I have been 
able to ascertain, any direct communication with each other; an indirect communi- 
cation, how'ever, is perhaps established through the system of interseptal canals. 
But each chamber normally communicates with two chambers of the annulus within 
it, and also with two of that which surrounds it, by large passages (shown in horizon- 
tal section in Plate XXIX. fig. 12, and in vertical section in Plate XXXI. fig. 2, c, c, 
and represented in perspective view in Plate XXX. fig. 4,/y’and g g), which traverse 
the annular septa ; of these passages there seems to be normally but a single one for 
eaeh pair of chambers thus to be brought into communication ; but I have frequently 
met with two, and occasionally three, one placed directly or obliquely above the 
other. Thus at each extremity of the oblong chamber, there are normally two pas- 
sages leading to two chambers of the annulus next internal or external to it ; but since 
to each of these chambers there may be two or even three passages, the total number 
at eaeh end may be three, four, five, or six. The variations as to this point of strueture 
which are presented in adjacent chambers, are shown in Plate XXXI. fig. 2. But 
since, on the other hand, each of the ‘ interpolated’ chambers communicates with 
only one chamber in the annulus next internal to it, there may be but a single passage, 
in place of two or more. 
4 D 2 
