TWO GIGANTIC TYPES OP ARENACEOUS EORAMINIEERA. 
735 
how, if the laying of this floor was effected by the junction of peripheral extensions 
formed around the extremities of independent ‘ radial tubes,’ its surface should possess 
the regular continuity by which we find it characterized. In the somewhat analogous 
case of Tubipora musica, it is well known that the transverse lamellae connecting the 
polype tubes at intervals, which are formed by lateral extensions from the several tubes 
whose vertical growth sustains an occasional interruption at those points, are far from 
exhibiting such regular continuity. And with the knowledge we have of the general 
indefiniteness and polymorphism that prevails among the various groups included under 
the Rhizopod type, it seems still less to be expected that the multitude of ‘radial tubes’ 
proceeding from one lamella of ParJceria should all simultaneously cease to extend them- 
selves longitudinally, and should give off lateral processes with such perfect uniformity 
that their coalescence should form a continuous lamella exactly concentric with the pre- 
ceding, though connected with it only by the hollow pillars which traverse the inter- 
space. On the other hand it is to be remarked that, as all the chamberlets of each 
lamella open freely on its exterior, the surface of that lamella, whilst as yet uninvested 
by a subsequent growth, may be presumed to have been covered by a continuous layer 
of sarcodic substance, in freest connexion with that which occupied its own labyrin- 
thic substance. And it would seem not improbable, therefore, that the continuous im- 
perforate floor of the superposed lamella should have been laid by the instrumentality 
of this layer upon its own surface ; which must then have been completely cut off by 
it from direct connexion with the surrounding medium. Such communication must 
thenceforth have been only maintained indirectly, through the ‘ stolon-processes’ occu- 
pying the ‘ radial tubes ;’ the extensions of which, spreading themselves out on the ex- 
ternal surface of the floor of the new lamella, would build up the walls of the chamber- 
lets, and would occupy their cavities, — becoming in their turn the base of a new sarcodic 
investment of what would then be the external layer of the sphere, which, in due course, 
would itself be enclosed by another lamella, of which the imperforate floor would be 
modelled on its own surface. 
20. The question naturally arises whether each sarcodic layer, when it had com- 
pletely floored over the interspace it may be supposed to have filled, continued to occupy 
that interspace ; or withdrew itself into the labyrinthic system of its own lamella, and 
then projected itself through the radial tubes into the labyrinthic system of the next. 
To this question no positive answer can be given, since no recent organism is known, 
in which any analogous arrangement exists. But the former of the two alternatives 
seems to me the more probable, on the following grounds. If the sarcodic segments 
were withdrawn from the interspaces, their place must have been taken by some' other 
material ; since that a vacuum should have been left, is of course inconceivable. How 
air could have found its way into them, is scarcely more conceivable ; since these organ- 
isms must have been living, like recent Arenaceous Foraminifera, on the bottom of the 
sea, probably at a considerable depth. If any direct communications had existed be- 
tween the interspaces belonging to different layers, and between those of the penulti- 
