Tasch: Growth and Form of Fossil Protists 445 



vacuoles of tintinnids, quantitatively they are a minor contributor to the food 

 economy of the sea.^ 



Thompson's explanation^ of the basket-shaped skeletal units of Dictyocha 

 envisioned 4 or more vesicles side by side in one plane and separated by a 

 "polar furrow." The radial spines normal to the main basket or lattice work 

 were interpreted to be uncompleted portions of a larger basket. This last 

 interpretation seems unacceptable in light of the work of K. Gemeinhardt.^^ 

 He demonstrated that adult individuals had a smaller skeleton fitted into the 

 larger one. In this instance, the inner set of radial spines were not the be- 

 ginnings of a still larger skeleton, but rather parts of the skeleton of a daughter 

 cell, and its appearance preceded division. Hovasse confirmed this finding, in 

 1932,^^ and noted that the new skeleton was a mirror-image of the old one. 



Thus, the opaline silica lattice work may be envisioned as derived by secre- 

 tion on a tiny sphere of protoplasm that had a vesicular surface. Open space, 

 ovate, elliptical, and polygonal skeletal configurations can then be readily 

 explained. The radial spines which confer a stellate appearance are most 

 Ukely adaptive modifications to sustain flotation when the single flagellum is 

 at rest. All other accessory spines and ornamentation, such as beads and pits 

 on the discs,*^ may constitute minor adjustments of specific gravity of the 

 skeleton that had selective value. 



In the evolution of the silicoflagellates there is a tendency to increase the 

 number of radial spines from 3 or 4, to 6, 8, and 10. That trend clearly de- 

 notes the adaptive value of particular skeletal modification. 



Miscellaneous Protists 



In this section, bacteria and lime secreting algae will be considered from the 

 special point of view of our discussion. Despite the frequency of pleomorphism, 

 there are three common or fundamental forms of true bacteria; spherical or 

 ovoid (coccus), rod-shaped {bacillus), and spiral (spirallum). Spherical forms 

 may grow in pairs, in fours, or in chains. Rods vary in configuration from 

 cylindrical to ellipsoidal with rounded-to -flattened ends. In young cultures 

 and favorable media, bacteria tend to "exhibit characteristic morphology,'' 

 whereas in senescence, there are a decrease in size and considerable form varia- 

 tion. Other factors influencing shape are: temperature and age of culture, 

 concentration of substrate, and composition of medium.^^ A barophilic 

 property (pressure-dependence) has also been reported. Near their threshold 

 of pressure-tolerance, cells of many bacteria grow into long filaments and 

 mutations are promoted.""''^ 



Bacteria are commonly about 0.5 fj, in size but range to 10 fx. Fossil bacteria 

 are generally identifiable by size, shape, and arrangement alone.*- However, 

 viable bacteria of Permian age have since been reported from the United States*^ 

 and from Germany (Dombrowski, 1960). In such instances, physiological 

 activities which distinguish modern bacterial species can also be studied in 

 ancient populations. 



The descriptive literature on lime secreting algae known as fossils is very 

 extensive. "^"^^ A good review of recent stromatolites and their ancient ana- 

 logues is given by Ginsburg.*^ Types of stromatoUte configurations include: 



