Till 
INTRODUCTION. 
in the successive chambers of the shell are produced by one or more stolons , 
forming short cords, by which the segments are connected, and for which 
a space or aperture is left in the wall of each chamber as each new mass 
of sarcode is coated with shell-matter, either by the internal mass or by 
the overlying layer — in the latter case, after the mode observable in the 
shell-secreting outer mantle of Cypraea and such-like mollusks. The 
pseudopodial pores also are left open by this external shell-forming layer, 
as in Polystomella, except where they become interrupted, as, for instance, 
at the margin of the shell, or where the divisions or septal walls of succes- 
sive chambers, or alar flaps, cross one another, as in Nummulites. This 
shell-growth also produces, in some instances, tubercles and ridges, as in 
Nodosaria ; and, in some thick- or double-walled chambers, canals or 
vascular hollows are left for an intermediate system of sarcode threads, 
finding their way to the surface from the inside, as in Calcarina and 
others. 
Those Foraminifera which have porous or tubuliferous shells, such as 
most of the above-mentioned kinds, are termed Perforata ; and, having 
often a subtranslucent and shining shell, they are also termed Hyalina 
or Vitrea. 
Some Foraminifera secrete an opake, homogeneous, white, calcareous 
shell, each chamber of which is laid on, tent-like, over its segment of 
sarcode, the edges resting on the last, or some previously-formed, portion 
of the shell. Being without pseudopodial tubules, these are termed Im- 
perforata, and, from the solid white appearance of their shell-tissue, 
Porcellanea. 
In these the sarcode protrudes either through one large terminal aper- 
ture (as in Miliola ), or through a series of small holes at the edge of a 
more or less discoidal compound shell, made up of concentric rings, some- 
times divided into chamberlets (as Orbiculina ), or consisting of circles 
of small chambers (as Orbitolites). 
Some kinds of Foraminifera (such as Lituola ) always mix up grains of 
sand, minute shells, or other extraneous particles with their calcareous 
coating. These are termed Arenacea. But some of the hyaline and per- 
forate kinds, such as Bidimina , become coarser and take up sand-grains in 
their shells as they grow old. So also some of the porcellaneous Miliolce 
become sandy under certain conditions. Therefore the Arenaceous Fora- 
minifera do not form so distinct a natural group as the others ; and, indeed, 
all three groups have interlinkings in this and in other respects. 
There are also Foraminifera (hat select sponge -spicules as the extrane- 
ous materials of their tests, either with or without other foreign particles. 
It is possible that some of these may have somewhat chitinous coverings 
