CHAP. 33] THE PRESERVED RECORD: PALEONTOLOGY OF PELAGIC SEDIMENTS 875 



valid for one group are not necessarily applicable to the other. The first of these 

 two groups is the Tripylea ( = Phaeodaria), with skeletons apparently formed of 

 an intimate mixture of silica and some organic substance: skeletons of this group 

 are not represented in the fossil record (Deflandre, 1960), though they are 

 preserved in a few Recent sediments deposited under exceptionally favorable 

 conditions (Fig. 8). The second group (comprising Spumellaria and Nassellaria) 

 has skeletons of opaline silica without organic admixture, and it is these 

 w hich are preserved in sediments. It may be mentioned that the Acantharia, 

 which have skeletons of strontium sulphate and are regarded as Radiolaria by 

 some workers, are not known with certainty to occur in either Recent or 

 ancient sediments. 



Because of their relatively large size (up to a few millimeters), the Tripylea 

 are often observed in plankton samples and have therefore been the subject of 

 a number of biogeographic studies (annotated bibliography of Riedel and Holm, 

 1957). But the Sphaerellaria and Nassellaria, with which we are concerned here, 

 are much more difficult to obtain from plankton samjDles and their biogeo- 

 graphy is consequently sparsely documented. It appears that the majority of 

 them are restricted in life to the upper few hundred meters of the water column, 

 though Haecker (1907) observed some living at depths of 400-5000 m. Popofsky 

 (1908, 1912, 1913) found some species of Sphaerellaria and Nassellaria to be 

 restricted to cold waters and others to warm waters, and the size of some widely 

 distributed species to vary according to the temperature of their habitat. 



H. Sponges 



Members of this group of sessile Metazoa are widespread on the ocean floor, 

 and contribute siliceous spicules of a variety of forms (Fig. 9) to j)elagic sedi- 

 ments. Sponge spicules can generally be distinguished from fragments of other 

 siliceous microfossils by their having a central canal. In deep-sea sediments these 

 spicules usually constitute only a minor proportion of the biogenous silica, 

 though they form a major constituent of some assemblages on shallower banks 

 and continental shelves. The present impossibility of identifying isolated 

 spicules limits their use in paleoceanographic interpretations. 



5. Interpretation of Quaternary Microfossil Assemblages 



Before attempting any interpretation of microfossil assemblages in Quater- 

 nary sediment cores, it is necessary to evaluate the fidelity with which distri- 

 bution patterns of species in Recent sediments reflect those of the organisms 

 living in the waters above. The most conclusive way to accomplish this is to 

 compare directly the distributions of living planktonic species with the distri- 

 butions of their skeletal remains at the sediment surface. For most planktonic 

 groups this is impossible at present because of lack of the necessary data, and 

 very few investigations have been made with this object directly in view. On 

 the basis apparentl}^ of little evidence, Murray (1897) categorically stated that 



