B 



Fig. 176 Pachytheca. A, transverse section with natural opening 

 through cortex ( x 12). B, algal filaments of medulla and inner 

 cortex ( X 240). Cj cortex with algal filaments ( x 60). D, cortex 

 showing degenerate algal threads in tube ( x 150). (After Lang.) 



much the same way as tJie algal water-biscuits now found in South 

 Australia. These range from tiny particles to thick bun-like forms 

 20 cm. in diameter, whilst in them are to be found the tube-like re- 

 mains of living species of Gloeocapsa and Schizothrix, Gloeothece 

 and Gloeocapsa are also known to form ooHtic granules in the neigh- 

 bourhood of Salt Lake City. The presence, however, of pebbles, or 

 the existence of a granular structure, does not necessarily involve 

 the presence of algae, and in some cases it is also possible that the 

 algae were merely included through chance. Pachytheca is a genus 

 from the Silurian and Devonian which possessed a medulla of inter- 

 twining tubes and a cortex composed of stout, septate, branched 

 algal filaments that radiate from the medulla to the periphery. Its 

 affinities are extremely uncertain and it may have been a free-roUing 

 alga of either salt or fresh waters (cf. Fig. 176). 



Dinophyceae, Bacillariophyceae 



There are a few imcertain fossils, very indistinct and not well 

 known, ascribed to the Flagellata and Dinophyceae. Recognizable 

 fossil diatoms are known from the Upper Jurassic, but they reached 



296 



