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



873 



E. Pteropods 



Some members of this group of pelagic molluscs have shells which are gener- 

 ally spirally coiled, conical or compressed-conical, up to several millimeters in 

 length (Fig. 5). Biogeographic studies of this group have been made by 

 Meisenheimer (1905), Tesch (1946, 1948) and Hida (1957). Characteristically 

 tropical, subtropical, boreal and ubiquitous species are known. The majority of 

 species live within the upper few hundred meters of the water column, but 

 there are some bathypelagic species living predominantly in depths greater 



lOOii 



Fig. 5. Pteropods from a Recent sediment 

 from the Tuamotu Archipelago. 



Fig. 6. Fish skeletal debris from a 

 South Pacific zeolitic clay. 



than 500 m. The contribution of pteropods (and heteroiDods, which have some- 

 what similar shells) to pelagic sediments is severely limited by the instability 

 of their aragonitic shells. They are found more commonly in Mediterranean and 

 Atlantic than Pacific sediments. This apparently reflects the shallower depth 

 and more rapid rate of sediment accumulation in the former. 



F. Fish Debris 



Fragments of bone, placoid scales and small teeth (Fig. 6) form a minor 

 constituent of all pelagic sediments, but there has been no reported attempt to 

 identify the fish they represent. 



G. Radiolaria 



Members of this group of Protozoa occur in all parts of the oceans, and 

 contribute significant amounts of opaline silica to sediments, especially in 

 tropical and temperate regions. Their tests, most of them 50-400 \l in diameter 

 or length, exhibit a great variety of shapes generally based on spherical or 

 helmet-shaped ground-forms (Fig. 7). In attempting to evaluate the literature 

 on Radiolaria, it is important to bear in mind that there are two profoundly 

 different, not closely related, groups of siliceous Radiolaria, and generalizations 



