TWO GIG- ANTIC TYPES OF ARENACEOUS FORAMINIFERA. 
727 
the level of the former, and would thus cause the solidified portions to project as ridges. 
—That the infiltrating material might more readily penetrate the substance of the de- 
pressed portions than that of the tubercles, will become evident when it has been shown 
that whilst the latter is the most solid portion of the fabric, being minutely labyrinthic 
throughout, the former is composed of a mere layer of labyrinthic structure, having 
large cavities beneath it (see § 17). 
7. Although I have spoken of the Material of which these fabrics are mainly com- 
posed as c sand,’ yet its composition is very peculiar. When a portion of a non-infil- 
trated specimen is treated with very dilute Nitric acid, a slight effervescence takes place, 
and the arenaceous particles fall asunder. But when these are treated with stronger 
nitric acid, by far the larger proportion of them is dissolved ; only a small residue 
remaining insoluble in the strongest acid. Hence it appears (1) that the Arenaceous 
particles are held together (as in many other Arenaceous Foraminifera) by a cement 
of Carbonate of Lime, which, however, forms no large proportion of the whole ; and (2) 
that, although Siliceous sand-grains do occur in small proportion, the principal part of 
the Arenaceous material is not siliceous, but (being soluble in moderately strong nitric 
acid) is probably Phosphate of Lime. This proves, in fact, to be the case ; a careful 
analysis, made under the direction of Mr. H. B. Brady, having given the following 
results : — 
Phosphate of Lime 59*7 
Carbonate of Lime 26-0 
Silica 9-0 
Iron and Alumina 0-9 
Magnesia, Manganese, Organic Matter, &c. . . . 4-4 
100-0 
8. In several of the specimens which have been partially or completely infiltrated, the 
chamberlets and sometimes even the large interspaces of the external layers are occupied 
by a substance which presents a green colour alike by transmitted and by reflected light : 
this probably consists of the Silicate of Iron, Alumina, and Magnesia, of which the 
ordinary green particles of the Upper Greensand Formation are composed, — these par- 
ticles, as Professor Ehrenberg* has shown, being really the chamber-casts of Foramini- 
fera whose calcareous shells have entirely disappeared. 
9. Internal Structure. — Proceeding now to the details of the internal structure of 
these singularly fabricated bodies, we shall find it convenient, as in the case of the dis- 
coidal Orbitolites (Philosophical Transactions, 1856, p. 194), to distinguish between the 
Nucleus and the Concentric Layers that surround it. 
10. The Nucleus , as shown in situ in Plate LXXII., and on a larger scale in Plate 
LXXIII. (divided transversely in fig. 1, and longitudinally in fig. 2), is composed of a 
series of chambers, c'-c 5 , which are laid end to end in a rectilineal direction ; the series, 
* “ Ueber der Griinsand und seine Einlauterung des organischen Lebens,” in Abhandlnngen der Konig- 
lichen Akademie der Wissencbaften, Berlin, 1855. 
