reproduction; evolution 295 



(Fig. 174), which seems to have affinities with the modern Schtzo- 

 thrix, 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 (Precambrian onwards) 



Much doubt has been thrown upon the authenticity of this 

 group, some v^iters regarding them as structures which originated 

 as diffusion rings ('liesegang' phenomena) in colloidal materials or 

 perhaps in calcareous muds. In the original description Walcott 

 suggested an affinity to the Myxophyceae, but as later workers 

 could only distinguish a purely mineral structure they suggested 

 the idea of diffusion phenomena. Discoveries of very comparable 

 algal concretions and laminations in the Bahamas, however, have 

 made it extremely probable that these structures had an algal 

 origin. Further good examples of these structures have since been 

 reported from the United States. Some examples of these types are 

 shown in Fig. 175. On the basis of Black's discoveries (1933) it may 

 be suggested that these structures were not necessarily formed by 

 deposition but that the algae collected and bound the sediment. 



rV:~\~>.:-.' 



Fig. 175 Stromatolithi. A, Weedia. B, Collenia. C, D, Cryptozoon. 

 E, Archaeozoon. F, Gymnosolen. (After Hirmer.) 



Porostromata (e.g. Girvanella, Sphaerocodium) 



These forms, which are most abundant in the Carboniferous, 

 have a recognizable microscopical structure, the threads often be- 

 ing arranged in a radiating fashion : they were probably formed in 



