268 REPRODUCTION, EVOLUTION, ETC. 



of intertwining tubes and a cortex composed of stout, septate, 

 branched algal filaments that radiate from the medulla to the peri- 

 phery. Its affinities are extremely uncertain and it may have been a 

 free-rolling alga of either salt or fresh waters (cf. fig. 169). 



There are a few uncertain fossils, ver\^ indistinct and not well 

 known, ascribed to the Flagellata and Dinophyceae. Recognizable 



B 



Fig. 169. Pachytheca. A, transverse section with natural opening through cortex ^ 

 ( X 12). B, algal filaments of medulla and inner cortex ( x 240). C, cortex with 

 algal filaments ( x 60). D, cortex showing degenerate algal threads in tube ( x 150). 

 (After Lang.) 



fossil diatoms are known from the Upper Jurassic, and there was 

 a very rich fossil diatom flora in the Tertiary, all the specimens 

 found being closely related to existing families and genera. 



Codiaceae 



Boueina (cf. fig. 170) is an unbranched form from the Lower 

 Cretaceous, whilst Palaeoporella (fig. 170), which is composed of 

 hollow cylinders or funnel-shaped bodies with slender forked 

 branches, the whole being two to fourteen millimetres long, comes 

 from the Lower Silurian. Dimorphosiphon, from the Ordovician, is 

 generally regarded as the oldest known member of the Codiaceae 



