TWO GIGANTIC TYPES OF ARENACEOUS FORAMINIFERA. 
733 
in the more solid fabrics. — It is difficult to conceive the means whereby the pseudo- 
podial filaments projected by the sarcode-body, which must have been the instruments 
employed to collect the sand-grains, were enabled to fit them together with a precision 
that could not be exceeded by the most dexterous mason, employed to build up angular 
stones of every variety of shape, with the smallest possible quantity of intervening 
cement, into a wall of uniform thickness and general regularity of surface. But a 
precisely parallel case occurs among existing Arenaceous Foraminifera , as I have else- 
where shown*. 
17. Notwithstanding the apparently well-marked difference between the structure of 
the layers now described, and that of the earliest layers represented in Plate LXXIII. 
figs. 1 & 2, we shall find this difference to consist almost entirely in the proportions of 
the component structure. The labyrinthic substance in the latter case presents itself 
in an almost rudimentary condition, the lamellae being very thin, whilst the interspaces 
between them are very wide, the entire thickness of the layers , which are composed of 
lamella and interspaces taken together, being about the same as elsewhere. The width 
of the interspaces, and the absence of the labyrinthic structure which elsewhere forms 
the ‘radial processes,’ makes the ‘radial tubes’ very conspicuous; and they are some- 
what more closely set than in the outer layers. The first trace of labyrinthic substance 
is seen in the wall of the Nucleus itself, alike in transverse and in longitudinal section 
(l, Z, figs. 1, 2, Plate LXXIII.); from this, as from a base, spring a number of hollow 
pillars, the ‘ radial tubes,’ whereon the first of the investing lamellae is supported, of 
which the cancellated structure is scarcely more developed ; and this again serves as a 
base for another set of radial tubes, that support a second lamella in which the can- 
cellated structure is somewhat more obvious. — In the transverse section (fig. 1), we see 
that the earlier layers do not pass completely round the nucleus, their disposition being 
somewhat excentric, as is very commonly the case with the earlier zones of Orbitolites 
(see Philosophical Transactions, 1856, pp. 217, 218); it will be seen, however, on refer- 
ence to Plate LXXII., that the regularly concentric arrangement is soon established in 
this direction. In the longitudinal section (fig. 2), however, the earlier lamellae are seen 
to be considerably less complete, enclosing but a small part of the length of the nucleus ; 
which does not seem to be encased at both its extremities, until four or five of such in- 
complete lamellae, each extending somewhat further than that which preceded it, have 
been formed upon the surface of the elongated primordial chambered cone. Proceeding 
further outwards, we find the concentric lamellae progressively increasing in thickness, 
in consequence of the augmented development of their labyrinthic structure, while the 
interspaces are proportionately narrowed ; and we see the labyrinthic system of one 
lamella occasionally putting forth irregular outgrowths, which cross the interspaces 
(usually clustering round the ‘radial tubes’), and become continuous with the laby- 
rinthic system of the succeeding lamella. Towards the outer part of this series of layers, 
which ends with the first thick lamella (l 1 , Plate LXXII.), not only does the thickness of 
* ‘ Introduction to the Study of the Foraminifera § 213, and Plate YI. fig. 41. 
