tend into the Recent. The genus Hystrichosphaeridium is most common in the 

 Cenozoic, although it occurs in the Mesozoic and Paleozoic as well. 



In summary, several factors emphasize the usefulness of the hystrichospher- 

 ids to petroleum geologists: (1) they are found in rocks from pre-Cambrian to 

 Recent; (2) they show characteristic differences between rocks of different ages; 

 (3) they show differences in form with respect to their relative distance from 

 reefs or ancient shores; and (4) they are found in many different lithologies. 



Chitinozoa (PI. 11 -IV, figs. 23, 24) 



As the name implies, the Chitinozoa are composed of chitinous material. 

 They occur in marine sediments as hollow membranes in various forms such as 

 tubes, flasks, urns, funnels, and bells. Their biological affinity is not known. Be- 

 cause of their occurrence in early Paleozoic rocks (Cambrian-Mississippian) , 

 they are especially valuable to the stratigrapher in the study of Cambrian and 

 Ordovician sediments that usually are void of other microfossils. 



Nannoconus (PI. 11 -IV, fig. 25) 



Another form of uncertain affinity is Nannoconus. These rock-forming, min- 

 ute calcareous organisms (size range 100-180 microns) are found in Jurassic 

 and Cretaceous marine sediments. They are cone-like, consisting of many wedge- 

 shaped elements built around a central canal. Their wide geographic distribu- 

 tion and restricted stratigraphic range make them useful. Bronnimann (1955) 

 has used them successfully in zoning Lower Cretaceous limestones in Cuba. 



Oligostegina 



Small, rock-forming calcareous spheres (size 50 microns +) known as 

 Oligostegina are found in Cretaceous rocks. Early workers thought they belonged 

 to the foraminifera, but the affinity of these small pelagic forms remains in doubt. 



Fragmenta 



Unidentified spheres, spines, plates, tubes, and other remains frequently oc- 

 cur in prepared residues. These microscopic fossils probably represent elements 

 from unfamiliar animals and plants. Future work on these unknown microfossils 

 undoubtedly will establish their relationships and perhaps they may prove to be 

 useful stratigraphic markers. 



SUMMARY The oil companies are viewing with interest 



the growing importance of micropaleontolog- 

 ical analysis, which includes the study of all types of minute fossils. Through 

 improved recovery techniques, and the recent trend toward studying plant micro- 



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