S4 
THE OCEAN WORLD. 
VI. Agathistega. — Animal formed of segments wound round ^ 
axis. Chambers formed round a common axis, each investing half tb e 
circumference. 
Fig. IS- Spirolocnliiia. 
The simplest form of Foraminifera is illustrated by Fig. 12 ( Orbuli # 
universa), which is a small spherical shell, having a lateral aperture 
the interior of which has been occupied by the living jelly, to whi°ij 
the shell owes its existence. In the second order, the shell (Fig. l^)’ 
Dentalina, communis, advances beyond this simple type by a proce^ 
of linear budding, the first cell being spherical, with an openi®# 
through which a second segment is formed, generally a little larg ff 
than the first. This new growth is successively followed by other 3 
developed in the same way, until the organism attains its maturity 
when it exhibits a series of cells arranged end on end, in a slight 
curved line. 
In the next group the gemmation takes a spiral bias, producing ^ 
nautilus shape which misled the earlier naturalists. In some caS^ 
all the convolutions are visible, as in Operculina (Fig. 14). In other* 
the external convolute conceals those previously formed, as in 
mulitis leniicularis (Fig. 15), Cassidulina (Fig. 16), Textile 
(Fig. 17), and Alveolina oblonga , d’Orbigny (Fig. 23), the latte r 
forming part of the eocene formation in the quartz and greysto^ 
rocks of the neighbourhood of Paris; one figure representing ^ 
shell entire, and the other a vertical section, while the small fig llJl! 
between represents it in 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 reverb 
as the segments are multiplied, as in Cassidulina (Fig. 16). - ' 
the succeeding group the new segments are arranged alternately 0(1 
