ELECTRON-MICROSCOPY OF FOSSILS — ^HAY 411 



As an example of the sort of study of shell ultiustnicture now 

 possible, consider the investigations on the protozoans known as Fora- 

 minifera carried out recently by Hay, Towe, and Wright.^ 



The Foraminifera have long been of particular interest to paleon- 

 tologists, since they are among the most common fossils, and are par- 

 ticularly useful in determining the age of strata. The Foraminifera 

 build shells called "tests" in two ways: (1) by gluing foreign par- 

 ticles together with silica, iron oxide, or calcium carbonate cement, or 

 (2) by constructing tests of crystalline calcium carbonate by 

 secretion. The first group of Foraminifera are commonly called 

 "arenaceous" and the second "calcareous." The calcareous Foraminif- 

 era may be subdivided into two major groups by the external appear- 

 ance of the test. One group contains forms that have glistening white 

 walls that are opaque. They superficially resemble china so closely 

 that they are referred to as porcellaneous Foraminifera. The second 

 group has walls that are translucent, so that the tests look as though 

 they are made of glass, and are termed "hyaline." 



There is another major difference between the porcellaneous and 

 hyaline Foraminifera ; the porcellaneous forms are also called "imper- 

 forate" since the wall is apparently solid and the protoplasm inside 

 the test can communicate with the exterior only through a large open- 

 ing at the end of the test known as an aperture. Tlie tests of the 

 hyaline Foraminifera are pierced by a number of minute pores, and 

 the tests are said to be perforate. The protoplasm inside the test can 

 communicate with the outside not only through a large aperture, but 

 through the myriad pores as well. 



Although porcellaneous Foraminifera are very common, especially 

 in the shallow-water deposits of the Atlantic and Gulf coasts of 

 the United States and are familiar to every micropaleontologist, 

 the nature of the porcellaneous wall has remained an enigma. Early 

 investigators described the walls as constructed of globular bits of 

 calcium carbonate, but modern workers have been unable to substan- 

 tiate this or to see any structure in the wall at all. Chemical analyses 

 have shown that the wall is made of impure calcium carbonate, with 

 magnesium atoms present at about 10 percent of the spaces in the 

 crystal structure that are usually occupied by calcium. In reflected 

 light some of the tests appear to glisten, indicating that the surface 

 must be smooth enough to reflect light. Other porcellaneous tests 

 belonging to the same species may be dull, but all are white and opaque 

 in reflected light. 



Viewed by transmitted light in a microscope, the tests of porcel- 

 laneous Foraminifera appear to be structureless, but do possess a 



" Hay, W. W., Towe, K. M., and Wright, R. C, Ultramlcrostructiire of some selected 

 foramlnlferal tests. Micropaleontology, vol. 9, No. 2, pp. 171-195, pis. 1-16. 1963. 



