286 
THE VOYAGE OF H.M.S. CHALLENGER. 
four Sub-families, genera and species exhibit minor peculiarities of structure, and even 
individual specimens are more or less affected in this respect by local conditions. This 
diversity of structure depends partly on the sort of foreign matter employed in the construc- 
tion of the test, and partly on the cement secreted by the animal for its incorporation. 
Notwithstanding a certain amount of selective power, the nature of the foreign 
material depends more or less on the character of the sea-bottom. Siliceous sand is the 
most common extraneous constituent; but on some sea-bottoms, as for instance in pure 
Globigerina ooze, siliceous grains are rare, and the dead shells of the smaller Foraminifera 
are used in their stead (PL XXX. fig. 13). In the tropics, siliceous sand is often replaced 
by the calcareous debris of the coral-reefs (PI. XXX. figs. 15-17); and in like manner the 
tests of Radiolaria and the frustules of Diatomacese are sometimes employed in considerable 
numbers where such organisms abound. The preference for sponge-spicules, broken or 
entire, as a building material, which is so conspicuous a feature of certain genera of the 
Astrorhizida:, exists in like manner amongst the Lituolide, though not to the same 
■extent (PI. XXXI. figs. 16, 17, &c.). 
But the physical characters of the test are also influenced, and perhaps to an even 
greater degree, by the mode in which the foreign constituents are combined. Under all 
circumstances the superficial portion of the sarcode, whether in the form of a distinct 
chitinous envelope or of a less completely differentiated layer, is the basis of the invest- 
ment. In some cases this of itself is sufficient to serve as the matrix for the arenaceous 
particles, but much more frequently inorganic matter is collected or secreted by the 
animal in quantity large enough to make a kind of cement or plaster, in which the foreign 
particles of whatever sort are embedded. The cement varies very much, both in quantity 
and chemical composition, but its principal constituent is generally either carbonate of 
lime or peroxide of iron. In rare instances silica or some siliceous compound is employed, 
either by itself or in conjunction with other mineral substances. 
In the Sub-family Lituolinai the test is constructed of coarse materials, and, as 
compared with the allied groups, the sand is proportionately in excess of the cement ; 
nevertheless, the cement is present in sufficient quantity to produce tolerably strong and 
substantial walls, though the exterior is rough and sometimes wears an unfinished 
appearance. The principal exceptions to the typical Lituoline structure are found 
amongst the Nodosariform species. In Reophax sabulosa the cement is deficient in 
quantity, and the test is thick and crumbling. In Reopliax nodulosa the incorporating 
medium is more or less siliceous, — sometimes to such a degree that large specimens, half 
an inch or an inch in length, preserve their form after all the calcareous and ferruginous 
constituents have been removed by means of strong acids, and still retain sufficient 
firmness to bear handling without injury. In Reophax membranacea the investment is 
little more than a brown membrane with sufficient calcareous matter and siliceous grains to 
