Silicoflagellata (PI. 11-1, figs. 2-4) 



Like the Coccolithaceae to which they are apparently related, the silico- 

 flagellates are small (25-80 microns) marine planktonic forms. They are widely 

 distributed in the present oceans. As in the case of the Coccolithaceae, some 

 workers refer them to the algae, whereas others place them within the Protozoa. 

 The silicoflagellates have siliceous skeletons composed of a simple combination 

 of spines, arcs, nets, and thin rings. They occur in Cretaceous and Tertiary rocks 

 and are especially abundant in diatomaceous sediments. Silicoflagellates are be- 

 ing used currently for correlation purposes and as indicators of marine condi- 

 tions. 



Dinoflagellota (PI. 11-1, fig. 5) 



Dinoflagellates are small (25-125 microns) planktonic organisms which are 

 considered by some workers to be algae and by others to be Protozoa. Their 

 wall cells or coats, which are cellulose, may be smooth and consist of two valves, 

 or they may be spiny and divided by furrows into angular plates. Their geologic 

 range is Jurassic to Recent. 



Representatives of this group are found in residues prepared for other acid- 

 insoluble organisms such as hystrichospherids, to which they may be related. 

 The dinoflagellates are rapidly becoming useful to the economic paleontologist. 

 Although this group is generally found in a marine environment, representatives 

 are known also from fresh and brackish waters. 



Desmidaceae (PI. 11-1, fig. 19) 



Desmids are algae (Chlorophyceae) whose acid-resisting cell walls contain 

 vertical pores through the innermost and median wall layers. Although most of 

 the desmids are unicellular, some have their cells united in filaments of various 

 lengths. They are found associated with free-floating algae in fresh water. Rep- 

 resentatives of this group are so different in shape and ornamentation that even 

 a general description here would be difficult. Fossil forms have been noted in 

 Tertiary and Recent deposits. They are useful to the economic paleontologist be- 

 cause they are limited to fresh water. 



Other Chlorophyceae (PI. 11-1, fig. 15) 



Other Chlorophyceae important to the paleontologist include the genus 

 Pediastrum of the family Hydrodictyaceae. This genus is characterized by a 

 coenobium (a colony of cells) usually composed of 4 to 64 cells. It is found in 

 fresh water sediments from Tertiary to Recent. 



Diatomaceae (PI. 11-1, figs. 6-11) 



Diatoms differ from other algal forms principally because their cell walls 

 are siliceous and consist of two closely fitting or overlapping halves. As in other 



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