TWO GIGANTIC TYPES OE AEENACEOUS EORAMINIFERA. 
745 
The spiral lamina itself has much the same dense opacity as is presented by older indi- 
viduals ; and the interior has, so far as can be made out, a loose, arenaceous texture, which, 
with a further deposit of cementing material, might be expected to give the sort of struc- 
ture we find at the axis of the mature shell. But in the absence of evidence from speci- 
mens in the intermediate stages, much importance cannot be attached to the characters 
of a single individual, the original features of which may have been greatly altered in 
the process of fossilization. 
34. In the fully grown examples, the first circuit of the ‘ spiral lamina' encloses a space 
of variable dimensions, — in some cases measuring from -$o to inch in diameter at the 
centre, and from ■§■ to J inch in length, more or less ; and the revolutions succeed each 
other with tolerable regularity at intervals of to -g L 0 - inch. From twelve to twenty 
revolutions are usually found in an adult specimen ; but twenty-five have been noticed 
in one instance, and doubtless even a larger number may occasionally be met with. 
35. The ‘ spiral lamina’ or ‘ primary skeleton,’ as it may be regarded, is composed of 
almost impalpable calcareous grains closely cemented. It is imperforate, and not more 
than from - ioo o 1° swo °f an inch in thickness. From this extreme tenuity it necessarily 
depends for support upon accessory structures. 
36. The space enclosed by its first revolution — constituting the central axis (Plate 
LXXX. fig. 2, c a) — is occupied by a mass of shell-substance somewhat resembling in 
general features a piece of fine sponge, but not quite uniform in its structure. Quite in 
the centre it assumes the form of a network of irregular anastomosing tubes, with the in- 
terspaces filled in with shell-substance to a greater or less extent ; but nearer the primary 
lamina, the irregularly disposed growths resolve themselves into a more definite series, 
and take a uniform direction. The outer portion becomes in fact the. commencement 
of a system of parallel columnar or tubular processes springing from the inner surface 
of the spiral lamina, and having their free ends directed inwards (Plate LXXIX. fig. 1, a, 
and fig. 2, a). This system of parallel tubuli may under favourable circumstances be 
traced throughout the course of the spiral lamina, except when interrupted by the occur- 
rence of septa ; and it forms, as well shown in the figures just referred to, a sort of lining 
to its inner surface. The two structures are in such close juxtaposition that they appear 
continuous ; and their physiological distinctness is only noticed in exceptional places, 
when a minute portion of the sarcode appears to have become entangled between them, 
leaving for a short distance the primary lamina free from the accessory skeleton (Plate 
LXXIX. fig. 2, b, and Plate LXXX. fig. 3, b). The nature and extent of the labyrinthic 
portion of the layers may be best understood from a longitudinal section taken on a line 
very near the periphery, as shown in Plate LXXIX. fig. 1 ; for whilst a central section 
presents all the layers in the same aspect, one taken near the exterior bears a different 
relation to each consecutive layer. Thus we have about the centre of the figure at c the 
transverse section of the parallel processes just described as lining the spiral lamina ; 
whilst at a is seen the lateral aspect of the same. Such a section leaves no doubt as to 
the tubular character of even the more compact portions of the labyrinthic system. 
5 G 
MDCCCLXIX. 
