fossils by their composition of silica or strontium sulfate. The tests are spheroid, 

 asteroid, or bell-shaped. Because there are numerous types of radiolarians with 

 very similar shapes, it is often difficult to distinguish species. Radiolarians, be- 

 ing pelagic, are especially useful for long-distant correlation. 



Calpionellidae 



The Calpionellidae are small (25-50 microns) calcareous, uniloculur, cup- 

 shaped forms found in Upper Jurassic-Lower Cretaceous limestones, and are 

 probably related to the Tintinnoidea. 



Porifera (PI. 11-1, figs. 23-26) 



Needle-shaped and stellate calcareous or siliceous sponge spicules, averag- 

 ing a few millimeters in length, are found in rocks from pre-Cambrian to Re- 

 cent. Although they are not used widely in economic paleontology, they should 

 be useful in microfossil work when better understood. 



Alcyonarian Spicules (PI. 11-1, figs. 27, 28) 



Alcyonarian spicules are the microscopic calcareous or horny skeletal re- 

 mains of some of the alcyonarian corals. They occur in Upper Cretaceous and 

 Tertiary sediments. 



Echinodermata 



The skeletal elements of the holothurians, known as sclerites (PL 11 -I, figs. 

 29-32), have forms such as plates, wheels, hooks, ladles, and anchors. These 

 microfossils and the echinoid spines promise to become useful to the micro- 

 paleontologist when they are more thoroughly understood. The Echinodermata 

 are marine animals with a geologic range extending from Ordovician to Recent. 



Ostracodes (PI. 11 -IV, figs. 1-5) 



Although ostracodes are not used in correlation work as extensively as the 

 foraminifera; however, they are being used for age determinations and for paleo- 

 ecological interpretations. They are small, bivalved Crustacea, occurring more 

 abundantly in marine environments than in fresh and brackish waters, and 

 range from 0.5 to more than 20 millimeters in length. The two valves, gen- 

 erally unequal in size, usually can be distinguished as right and left. Ostracodes 

 are found in rocks of the Ordovician to Recent. 



The ostracodes are especially useful in the nonmarine beds where foramin- 

 ifera and other marine microfossils are absent, and are found most commonly in 

 shales, marls, and limestones. Because many genera and species have restricted 

 geological ranges and wide areal distribution, they serve as index markers and as 

 paleoecological indicators. 



220 



