RHIZOPODA—FORAMINIFERA, 85 
forming a spiral. Chambers disposed alternately along two or three 
axes, also non-spiral. 
VI. Agathistega.—Body composed of segments wound round an 
axis. Chambers formed round a common axis, each investing half 
the circumference. 
One of the simplest forms of Foraminifer is illustrated by Fig. 17 
(Orbulina universa), which is a small spherical shell, having a lateral 
aperture, the interior of which has been occupied by the living 
sarcode, to which the shell owes its existence. In Dentalina subarcuata 
the shell (Fig. 18), advances beyond this simple type by a process of 
linear budding, the first cell being spherical, with an opening through 
which a second segment is formetl, generally a little larger than the 
first. This new growth is successively followed by others developed 
in the same way, until the organism attains its maturity, when it ex- 
hibits a series of cells arranged end on end, in a slightly curved line. 
In the next group the gemmation takes a spiral form, producing 
the nautilus shape which misled the earlier naturalists. In some 
cases all the convolutions are visible, as in Opercudina (Fig. 19). In 
others, the external convolutions conceal those previously formed, as 
in Mummutlina perforata (Fig. 20), Textularia variabilis (Fig. 22), 
and Alveolina ovoidea, d’Orbigny (Fig. 16), the latter of its natural 
size. 
In the fourth group the shell is spiral, with the chamber equilateral, 
with a larger and smaller side; the position being alternately reversed, 
as the segments are multiplied, as in Cassidulina levigata (Fig. 21). 
In the succeeding group the new segments are arranged alternately 
on opposite sides of the central line, as in Zextularia variabilis 
(Fig. 22), thus forming two alternating non-spiral parallel segments, 
each connected by a single orifice. 
The sixth family differs entirely in appearance and structure from 
the other Foraminifera. The shells are more opaque than those 
met with in the other orders, having a resemblance to white por- 
celain, and present a rich amber-brown hue when viewed by trans- 
mitted light. They are more or less oblong, each new segment being 
nearly equal to the entire length of the shell, so that the terminal 
orifice presents itself alternately at its opposite extremities, some- 
times in one uniform plane, as in Spiroloculina depressa (Fig. 23), 
and Fotala (Fig. 24). At other times each new segment, instead of 
being exactly opposite each other, is a little on one side. 
This classification of d’Orbigny must now yield to that proposed 
by Dr. Carpenter in his work before referred to. He divides the order 
