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DE. W. B. CAEPENTEE AND ME. H. B. BEADY ON 
of which the transverse section is somewhat elliptical, having apparently commenced at 
the small extremity, and having very gradually widened until it ended abruptly at d*. 
The chambers are separated by septa, each composed of a single layer, which are ex- 
tremely sinuous, like those of many Ammonites. Of the mode of communication between 
the chambers, I am unfortunately not able to speak ; for although, by a happy accident, 
the sections of the only two uninfiltrated specimens yet examined, passed through the 
nucleus in such a manner as to lay it open transversely in one and longitudinally in the 
other, the portions of the septa traversed by the aperture are not exhibited in either — 
both the chamber-walls and the intervening partitions appear so perfectly homogeneous 
in texture, even under a magnifying-power of 80 diameters, that I was at one time in- 
clined to regard them as composed of proper shell-substance, corresponding with that of 
the Porcellanous Foraminifera , and free from any admixture of arenaceous particles, — 
its soft and friable texture bearing a close resemblance to that of the fossil Orbitolites 
of the Paris Basin. As the like appearance, however, is presented by the walls of the 
* radial tubes,’ which I have found to have the same Arenaceous composition as the re- 
mainder of the fabric (§ 14), I am now disposed to believe that the chamber-walls of 
the Nucleus are not exceptional in any other respect, than in the fineness of the parti- 
cles by the aggregation of which they have been built up. 
11. The general aspect of the Concentric Layers enclosing the Nucleus, as presented 
by a section whose plane passes through the centre of the sphere (Plate LXXIL), bears 
some resemblance to the median plane of an Orbitolite laid open by the removal of one 
of its superficial layers. (See Philosophical Transactions for 1856, Plate V., fig. 6.) 
But this resemblance diminishes on a closer comparison ; and disappears entirely when 
the details of the structure are examined with adequate magnifying-power. For whilst 
in Orbitolites the vacuities are chambers , symmetrically arranged in annular series, and 
communicating with each other by a regular system of circular galleries and radiating 
passages, which traverse the intervening solid shell-substance that forms the walls of 
those chambers, — the vacuities in Parkeria are merely irregular interspaces left between 
successive lamellae, each of which is composed of a 4 labyrinthic’ or cancellated sub- 
stance, made up of minute chamberlets separated by irregularly-disposed partitions, but 
freely communicating with each other ; and these vacuities are traversed by radiating 
tubes, which establish a direct communication between the ‘ labyrinthic system ’ of each 
layer, and that of the layers internal and external to it. Each of the concentrically 
spherical lamellae of labyrinthic substance , together with the interspace (traversed by 
* There is probably considerable variety in the disposition of the chambers of the Nucleus. In some speci- 
mens from the Isle of Wight (contained in the British Museum), "which, by the kindness of Mr. Woodward, I 
have had the opportunity of minutely examining since the above was written, the chambers are more numerous, 
and the axis of growth is not rectilineal but spiral ; and in the largest of these specimens the spire actually 
turns back on itself. The precise correspondence in structure between the Concentric Layers of these specimens, 
and those of the specimens described in the text, leaves me in no doubt that the direction of the axis of growth 
of the Nucleus has no essential significance. 
