ORDER I FORAMINIFERA 19 
The test may be either unilocular, enclosing a single cavity (Jlonothalamia), 
or it may be divided by septa into a greater or lesser number of chambers 
(Polythalamia). Multilocular tests all grow from a single primordial chamber, 
which is either spheroidal or elliptical in contour, and augment rapidly by 
means of the successive apposition of new chambers, each a little larger than 
the first, and each being applied in turn against the general pseudopodial 
aperture of the preceding. All the chambers communicate with each other by 
means of pores through which the sarcode is protruded. According as the 
chambers are applied against one another in a linear series (Sfichostega), in a 
spiral (/elicostega), in concentric rings (Cyclostega), in double or triple alterna- 
tions of either straight (Hnallostega) or spiral rows (Hntomostega), or envelop 
one another in irregular coils disposed in from two to five different planes 
(Agathistega), a great variety of forms may be produced. These differences 
in external form, together with peculiarities in the mode of growth, were 
employed by Alcide d’Orbigny as a basis for the first detailed system of 
classification of the Foraminifera that has been devised. For specific diag- 
noses, the relative size and shape of the shell, and varieties of ornamentation, 
such as bands, lips, protuberances, points, spines, and the like, are of greatest 
significance. 
In certain Foraminifera which secrete calcareous shells (Nwinmulinidae, 
Miliolidae, Lagenidae, Orbulina) a peculiar dimorphism has been observed, in that 
while some individuals have an extremely 
large primordial chamber (megasphere), A 
others, although differing in nowise in 
unre one and ornamentation, are 
characterised by an extremely minute 
primordial chamber (microsphere, Fig. 4). 
Those individuals which possess mega- 
spheres are almost invariably much 
inferior in size to those having micro- race 
spheres. De la Harpe refers this pheno- Biloculina_ Bradyi, Schlumb. Recent; Bay of 
menon to sexual differences ; Munier- PAY... Sal ofm with mesaaphern. |B Lage 
Chalmas and Schlumberger claim, on 
the other hand, that during the growth of those forms characterised by 
microspheres, the megaspheres, which are in all cases originally present, 
become absorbed and are replaced by an increased number of smaller 
chambers. Van den Broeck argues against these hypotheses, and attempts 
to explain dimorphism as the result of different modes of reproduction (fission 
and budding). 
The finer structure of the shell or test, which has been thoroughly 
investigated by Carpenter and Williamson, is of great importance in the 
classification of the Foraminifera. 
Chitinous tests are as a rule single-chambered (monothalamous), imper- 
forate, and are provided with a single large-sized oral aperture.  Silicious 
B 

Ehrenberg, C. G., Mikrogeologie, 1854, and Abhandlungen der Berliner Akademie, 1839. 
Schultze, Max, Ueber den Organismus der Polythalamien, Leipzic, 1854. 
Carpenter, W. B., Introduction to the Study of the Foraminifera, Ray Society, 1862. 
Reuss, E. A., Numerous Reports in Sitzungsberichte der Wiener Akademie, from 1860 onwards. 
Schwager, Conrad, Saggio di una Classificazione dei Foraminiferi, Bollet. Comitato Geol., 1876. 
Brady, W. B., Monograph of Carboniferous and Permian Foraminifera, Palaeontograph. Soc., 1876. 
Brady, W. B., Report on the Foraminifera, Scientific Results Challenger Exped., Zoology, XI., 1884. 
