868 RiEDEL [chap. 33 



of deep-sea paleontology is the elucidation of the history of the oceans on the 

 basis of biogenous components of sediments. The several steps toward this 

 goal are: 



(1) Mapping the distribution and abundance of the skeletons of pelagic 

 organisms in Recent deep-sea sediments, and determination of the fidelity with 

 which these thanatocoenoses reflect the biocoenoses of the overlying water. 

 The more fundamental investigation of the relation of species of organisms to 

 present-day physical and chemical oceanographic parameters is the province 

 of the biogeographer. 



(2) Determination of ages of various levels in sediment cores, or at least the 

 establishment of successive isochronous levels in the cores, 



(3) Comparison of isochronous fossil assemblages from many localities over 

 large areas, and interpretatioa (in terms of oceanic conditions) of observed 

 patterns in the distribution of species and assemblages. 



4. Shelled Pelagic Micro-organisms and Their Distribution 



The principal groups of organisms contributing skeletal material to pelagic 

 sediments are: 



1. Plants (restricted in life to the photic zone) 



a. with calcareous skeletons 



Coccolithophorids 



b. with sihceous skeletons 



Diatoms 

 Silicoflagellates 



2. Animals 



a. with calcareous skeletons 



Foraminifera 



Pteropods 



Fish (phosphatic remains) 



b. with sihceous skeletons 



Radiolaria 

 Sponges 



A. Coccolithophorids 



These organisms consist in general of a spherical or ellipsoidal protoplast of 

 the order of 5-50 [j, in diameter surrounded by a layer of joined or separated 

 circular, elliptical or angular calcareous plates (coccoliths, rhabdoliths, penta- 

 liths, etc.. Fig. 1). After death, the spherical tests (coccospheres, rhabdospheres, 

 braarudospheres, etc.) usually disintegrate into their component elements, 

 though they are occasionally preserved intact in sediments. Hentschel (1936) and 



