228 
THE VOYAGE OF H.M.S. CHALLENGER. 
as our knowledge undoubtedly is, the physical characters of the investment afford the 
best, indeed almost the only basis at present available for the classification of the group. 
Our acquaintance with the distinctive characters of the genera and their limits of 
variation is in like manner fragmentary and insufficient ; and it is more than probable 
that some of the forms at present accepted as independent types may be found, on the 
comparison of specimens obtained from fresh localities, to be nothing more than specific 
or varietal modifications belonging to the same generic series. 
The sequence of forms resulting from the successive modifications of the conspicuous 
characters of the test is less easily traced in the Astrorhizida: than in some other 
families of Ehizopoda, owing to the very elementary structure of many of the types. 
Perhaps the simplest conceivable sort of arenaceous investment is exemplified by the 
genus Psammosphcera, which has a globular test composed of coarse sand-grains firmly 
cemented together. It presents no general aperture, in the ordinary sense, and has no 
membranous lining, but the sarcode contained in the cavity communicates with the 
exterior by interstitial openings at points not filled with cement. The polythalamous 
condition, if such it may be called, of the same typical structure, is found in the genus 
Sorosphcera, and consists of a number of such spheres of uniform size, adhering by their 
exterior surfaces but unconnected by any definite sarcode stolons. In the absence of true 
stoloniferous passages Sorosphcera is more like a colony of monothalamous organisms than 
a single polythalamous test. 
Saccammina, in its simpler recent forms, presents a step in advance of Psammosphcera, 
for not only is the shell-wall more compactly built, and under ordinary circumstances 
imperforate, but the test has a distinct general aperture. When polythalamous, the 
chambers are either fusiform and united by stoloniferous tubes, or they are arranged side 
by side in colonies, with the orifices directed outwards. 
Storthosphcera and Pelosina, two genera of the sub-family Astrorhizina, present 
nearly parallel characters to Psammosphcera and Saccammina, but the firm, hard, 
arenaceous tests of the latter genera are replaced by an investment of fine sand and mud, 
almost devoid of cement, with smooth interior, and, in many cases, with a chitinous 
lining. The superficial orifices in Storthosphcera differ from those of its isomorph in being 
placed in little, irregular, pointed projections at various points of the periphery. 
Amongst the Pilulinina; the genera Pilulina and Technitella are also characterised 
by globular or oval tests, but they are constructed of masses of sponge-spicules, felted 
together, the interstices being filled with loose sand in place of calcareous cement. The 
distinction between these two types rests only on the shape of the test and the nature of 
the orifice. 
Thus it will be seen that in the genera Psammosphcera and Storthosphcera, Saccam- 
mina, Pelosina, Pilulina, and Technitella, the test consists typically of a spherical or 
oval chamber ; that in the first two interstitial orifices take the place of a general aperture, 
