When enlarged under the microscope and viewed by 

 transmitted light, foraminiferan shells of certain spe- 

 cies look like snail shells. ( West Germany. Kurt 

 Herschel ) 



isms. The protoplasm extends not only through the 

 pores but out of the mouth of the shell as well, pour- 

 ing out in all directions and flowing over the surface 

 of the shell. In the pseudopods, granules can be seen 

 streaming constantly toward the tips and then return- 

 ing along their outer edges. 



In contrast to the predominantly fresh-water ame- 

 boid protozoans just discussed, forams are almost ex- 

 clusively marine. Most of the species move about on 

 the ooze of the muddy bottom or attach themselves 

 loosely to debris on the ocean floor, usually in shal- 

 low waters but sometimes at depths of even 18,000 

 feet. Of more than twelve hundred living species, 

 only about twenty-six are pelagic and float in the 

 surface waters of the seas, mostly in the warmer 

 parts of the world, where the high alkalinity of the 

 water facilitates the extraction of calcium carbonate 

 from the sea. But these are the most prolific of the 

 forams, and when they die their innumerable shells 

 fall in a steady rain to the ocean floor, contributing 

 about 65 per cent by weight to the gray mud known 

 as "Globigerina ooze," from the genus of forams 

 that predominates in its formation. The most com- 

 mon foram species in the ooze is Globigerina biil- 

 loides, but shells of other species of Globigerina are 

 well represented, as are other foram genera, other 

 shelled ameboid protozoans, and especially the skele- 

 tal parts, called coccoliths (p. 23), which make up 

 nearly 30 per cent by weight of the ooze. Globigerina 

 ooze occupies nearly fifty million square miles of the 

 deep-sea bottom. Below fifteen thousand feet, how- 

 ever, the lime content of the ooze begins to thin out 

 because the calcite shells of the common foram spe- 

 cies become dissolved, and below eighteen thousand 

 feet calcareous shells are rare. 



The presence of Globigerina in fossil beds has 

 been used as an indication that the beds were de- 

 posited originally at a depth of between about three 

 thousand and twelve thousand feet. Opercidina shells 

 indicate a depth of less than 180 feet. The rate of 

 deposition of Globigerina ooze has been calculated, 

 for some areas, to be about four-tenths of an inch in 

 a thousand years. Though this is a rate in modern 

 times, it gives those who can comprehend such stu- 

 pendous figures some idea of the time it must have 

 taken to deposit the marine beds that, when uplifted, 

 form such great chalk formations (as much as 90 

 per cent calcium carbonate) as the white cliffs of 

 Dover in England, the chalk beds of Europe, and 

 the thousand-foot-deep chalk beds of Mississippi and 

 Georgia in the United States. Modern species of 

 forams are for the most part just visible to the naked 

 sye C^o of an inch), but many are of the size of a 

 pinhead and the largest one has a long, slender, tube- 

 like shell that may be 2 inches long. In geological 

 times past, when forams were more abundant than 

 at present, some members of the genus Nummiilites 

 had shells several inches across. Many large forms 

 flourished on the sea bottom in Tertiary times, and 

 their fossil shells, mostly about as big and flat as a 

 United States quarter-dollar, can be seen in lime- 

 stone now exposed in Asia, in the Alps, and also in 

 northern Africa, where such limestone was used to 

 build the pyramids of Gizeh, near Cairo. 



Limestones are produced instead of chalk when 

 foraminiferan ooze is deposited in waters close 

 enough to shores to become admixed with deposits 

 washed in from the land. The gradual change in 

 foram species from Tertiary times to the present 

 makes their shells very valuable as index fossils for 

 paleontologists trying to determine the age of various 

 sedimentary rocks. And because of their minute size 

 foram shells can be recovered undamaged, from 

 rocks far below the surface, in the borings made by 

 oil-well drills. By comparing the species of shells 

 brought up from different levels with species from 

 layers known to be oil-bearing, paleontologists are 

 able to direct oil-well drilling operations. 



A detection scheme on a much grander scale is 

 now unfolding in many laboratories around the 

 world, where foram shells are being used in studies 

 of world-wide glaciation, for piecing together the jig- 

 saw puzzle of the evolution and distribution of ani- 

 mals, and for understanding long-term climatic 

 trends. 



Globigerina biilloides has a shell (about ^4.t of an 

 inch) of spherical chambers spirally arranged and 

 perforated by many pores. When the foram is alive 

 and near the surface the shell is covered with long, 

 needle-like spines; these dissolve away when it later 

 falls to the bottom and is found in the ooze. The 

 protoplasm is said to be a rosy pink color when seen 



30] 



