FORAM/NIFERS. 
557 
straight line or in a spiral, and the coils of the spiral may or may not be in the 
same plane; or, again, the segments may form alternately on each side of a 
long axis. 
Polymorphina. 
a, b, c, From different aspects 
(magnified). 
In Polymorphina communis the segments are combined in a somewhat 
obscurely spiral arrangement. In the Foraminifera group a number of forms 
have arisen exhibiting an extensive series of 
variations on a few simple types, and show¬ 
ing transitions between forms which at 
first seemed distinct. The majority of 
species live at the bottom of the sea, but 
some are pelagic, and occur in abundance on 
the surface. Among the latter, Globiger- 
ina is one of the most widely distributed. 
Its shell is about one-fortieth of an inch in 
diameter, and usually composed of seven globular chambers, arranged spirally in 
such a manner that all are visible from above, but only the last four from below. 
Each chamber opens by a crescentic orifice into the depression in the middle of the 
lower surface. Perfect specimens bristle with long slender spines. The pores 
afford passage to the pseudopods which stream along the spines. In life, the 
shell is sunk in the midst of a bubbly sphere of protoplasm, which serves as 
a float. The investigations of deep-sea expeditions have brought to light 
the fact that the floor of the ocean, at depths between five hundred and 
two thousand five hundred 
fathoms over vast areas, be¬ 
tween 110° north and south 
of the Equator, is formed of a 
pinkish white mud, containing 
on an average about 60 per 
cent, of carbonate of lime. 
The presence of this material 
is mainly due to shells of 
Foraminifera, especially Glob- 
igerinidce, and, to a small 
extent, to the remains of 
minute pelagic algm, known 
as coccospheres and rhabdo- 
spheres; the broken fragments 
of the latter in the shape of 
shells of Globigerina (much magnified). d!scs and rocls bein S termcd 
coccoliths and rhabdoliths. 
Over the greater part of the floor of the Atlantic, and over immense tracts in the 
Western Indian Ocean and Pacific, over areas comprising in all about fifty millions 
of square miles, the ocean-bed is formed of Globigerina ooze. Chalk is mainly 
composed of the skeletons of Globigerinidw, coccoliths, etc., and, in fact, resembles 
Globigerina ooze. 
The question whether the Globigerinidw, which make up the bulk of the 
