302 the algae 



Phaeophyceae 



The principal fossil form ascribed to this group, Nematophyton^ 

 has now been removed to a new group, the Nematophytales (see 

 below). 



Rhodophyceae 



The Melobesiae are represented from the Cretaceous upwards 

 by species of Archaeoltthothamnion, DermatoHthon, Ltthothamnion, 

 Lithophyllum and Gonioltthon, some of them only being distin- 

 guished with difficulty from Hving forms. The Corallinaceae are 

 also represented in the Cainozoic by extinct members of present 

 living genera. There are a large number of forms assigned to an 

 extinct family, the Solenoporaceae, which existed from the Ordo- 

 vician up to the Triassic, but neither their structure nor their 

 systematic position has been completely estabUshed. They formed 

 nodules from the size of peas up to several centimetres in diameter 

 in which the cells were arranged like those of a Lithothamnton 

 although the cross walls were not well marked. Conceptacles have 

 been described in these plants and were presumably associated 

 with reproduction. 



The number of genera of different fossil algae that have so far 

 been recorded from geological strata is shown in Table 4. This 

 gives an indication of the age of the different groups and some con- 

 ception of their development. 



Nematophyceae: Nematophytales. Figs. 184, 185 



Two genera are now grouped in this assemblage which was estab- 

 lished by Lang (1937), and although he regards these forms as land 

 plants, nevertheless they have so many features in common with 

 the algae that it is felt proper to include them here. It is perhaps 

 almost too speculative to suggest that they represent transmigrant 

 forms, but it would appear that they must either be regarded as 

 highly developed algae which adopted a land habitat, or else as the 

 most primitive of all true land plants. The two genera agree closely 

 in their morphological structure, and although they are both fre- 

 quently found associated with each other in the Devonian rocks 

 die two structures have not yet been found in organic connexion. 



