FORAMINIFERS. 



557 



PolymorpMna. a, b, c, From different aspects 

 (magnified). 



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. 



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 

 011 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 algae, known 

 as coccospheres and rhabdo- 

 spheres; the broken fragments 

 of the latter in the shape of 

 discs and rods being termed 

 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 Globigerinidce, which make up the bulk of the 



SHELLS OF Globigerina (much magnified). 



