872 BiEDEL [chap. 33 



calcareous and others formed of agglutinated foreign grains, are widespread 

 but at present form a quantitatively minor constituent. However, planktonic 

 Foraminifera have not always formed a major constituent of sediments — they 

 first became abundant during the Cretaceous. The sparse benthonic foramini- 

 feral assemblages in pelagic sediments are relatively uniform in species- 

 composition, compared to assemblages of planktonic forms (Phleger, Parker 

 and Peirson, 1953). 



Planktonic foraminiferal species at present inhabiting the world's oceans 

 number only about 30-50, a surprisingly small total when compared to the 

 hundreds of described Recent species in such groups as the diatoms and Radio- 

 laria: the possibility should be borne in mind, however, that the large number 

 of species described in some groups may be to some extent a result of over- 

 splitting, not truly reflecting the number of non-interbreeding units. Murray 



I mm 



Fig. 4. Some Recent planktonic Foraminifera. 



(1897) noticed that the Foraminifera obtained in plankton tows in high latitudes 

 are different from those of temperate and tropical waters, and a few investi- 

 gators have since made more detailed biogeographic studies on this group. 

 Schott (1935) mapped the distributions of living planktonic Foraminifera in 

 the equatorial part of the Atlantic Ocean and noted that at least some of the 

 species showed distribution patterns apparently related to surface water masses 

 as defined by temperature and chemical characteristics. Be (1959), investigating 

 the Foraminifera in the upper 200-350 m of water in the North Atlantic over 

 an area bounded by 35-44°N, 55-65°E, found the biocoenoses to differ in the 

 three water masses represented (the slope- water in the north, the Gulf Stream 

 and the Central Atlantic water of the Sargasso Sea). Most of the nineteen species 

 occurring in this area were found throughout the region, but groups of species 

 separated as cold-tolerant and warm-tolerant differ in relative abundance from 

 one water mass to another. Bradshaw (1959) found it possible to group the 

 twenty-seven species of planktonic Foraminifera which he found in the North 

 and Equatorial Pacific Ocean into four faunas (subarctic, transitional, central 

 and equatorial-west-central) with limits corresponding in a general way with 

 those of the surface water masses. Many of the species occur in two or more 

 of his faunal regions, but zones of relative abundance are more restricted. In 

 Chapter 31 of this book Ericson discusses the interrelation between water 

 temperature and coiling direction of the planktonic foraminifer Globorotalia 

 truncatulinoides . 



