REPORT ON THE FORAMINIFER A . 
131 
and cannot be mistaken. The filmy shell or section is never really transparent, however 
thin ; and it has a uniform yellowish or light brown tint, very distinct from the nearly 
I colourless perforate structure exhibited under similar circumstances by the vitreous types. 
There are, however, numerous modifications of the typical structure, some of which are 
hereditary and characteristic of species, whilst others depend in part at least upon external 
influences. The most important of these arise from the tendency evinced under certain 
conditions to incorporate sand with the calcareous matter of the shell-wall, and the con- 
struction in such cases of a composite or arenaceous test, in place of the normal homo- 
geneous shell. In some species {Nubecularia lucifuga, PI. I. fig. 12, &c., Miliolina 
agglutinans, Miliolina crassatina, and Miliolina triquetra, PI. VIII. figs. 5-10) very 
coarse sand-grains are so employed, and the resulting test is indistinguishable in external 
texture from the rougher Lituolce ; in others (Planispirina celata, PI. VIII. figs. 1, 2) 
a uniform fine sand is the material selected, and the exterior corresponds more nearly 
with that of the Trocliammince. Nevertheless in all cases, however thick the sandy 
incrustation, there is a distinct, imperforate, calcareous shell, of the typical porcellanous 
structure underneath, immediately surrounding the animal. This may be easily traced in 
either the transverse or longitudinal section of the test. In the sandy forms the mouth is 
usually free from incrustation, and is encircled by a smooth, white, shelly rim. 
In brackish water, where the supply of earthy salts in solution is smaller than in the 
open sea, the chemical and physical characters of the shells of such species as survive the 
changed conditions are considerably modified. They become by degrees less calcareous as 
the water grows less saline, until eventually a point is reached at which the investment is 
little more than a chitinous or horny membrane, strengthened by the incorporation of 
minute siliceous grains, but containing so little carbonate of lime that it is scarcely altered 
by treatment with acids. 1 
A still more remarkable modification occurs in specimens from the abyssal depths of 
the North Pacific explored during the Challenger cruise. A few Miliolce, from soundings 
> taken at a depth of 3950 fathoms (about four miles and a half), with somewhat inflated 
segments, scarcely distinguishable in form from young thin-shelled specimens of a common 
littoral species, were found to be unaffected by treatment with acids, and upon further 
examination it became apparent that the normal calcareous shell had given place to a 
delicate, homogeneous, siliceous investment. Whilst immersed in fluid the shell-wall had 
the appearance of a nearly transparent film, and when dried was at first somewhat 
iridescent. The parietes of some of the specimens were so thin that they collapsed on 
j drying, but the stronger shells retained their form, and became white and more or less 
opaque by exposure to the air. 
Of the essentially imperforate nature of the shell throughout the Family there is no 
1 Miliolina ( Quinqueloculina ) fusca. H. B. Brady, On Brackish-water Foraminifera. Ann. and Mag. Nat. Hist., 
1870, ser. 4, vol. vi. p 286, pi. xi. fig. 2. 
