266 REPRODUCTION, EVOLUTION, ETC. 



Phaeophyceae. Whatever the sequence of events, it is quite clear from 

 the structure of the earHest fossils that considerable evolution had 

 taken place long before their time. 



FOSSIL FORMS 



In this section it is merely proposed to give an outline of the 

 different fossil forms that have been ascribed to the various groups, 

 but it is not intended to provide a detailed description in every case 

 so long as the types of structure represented among these fossil 

 algae have been adequately portrayed. It must be realized that 

 many of the early forms that have been ascribed to the algae are 

 relatively unknown because of the poor preservation, and further 

 examination of new specimens may mean that they will have to be 

 removed from the algae. For this reason it must be emphasized 

 that there are a number of doubtful forms from the lowest strata 

 which can only be tentatively assigned to the algae. 



Cyanophyceae 



Among the unicellular forms a fossil w^hich has been related 

 to the Chroococcaceae is recorded from the 

 Ordovician, It is called Gloeocapsomorpha 

 and is a colonial form with cells that were 

 apparently enclosed in a jelly, and whilst it 

 may have affinities with living colonial forms 

 it is usually placed in a group called the 

 Protophyceae. Another plant of Middle 

 Cambrian age, Marpolia spissa (fig. 167), 

 which seems to have affinities with the 

 modern Schizothrix, is also best relegated to 

 the Protophyceae. Marpolia was represented 

 by branched filaments which were probably 

 composed of a trichome enclosed within a 

 gelatinous or cartilaginous sheath. 



Spongiostromata (Precambrianto present day). 



Much doubt has been thrown upon the 

 authenticity of this group, some writers re- 

 garding them as structures which originated ^-^ ^^^^ Marpolia spissa 

 as diffusion rings ("liesegang" phenomena) ( x 49-5). (After Walton.) 



