Chapter XI 



FOSSIL FORMS, REPRODUCTION, 

 EVOLUTION 



Fossil Forms 



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

 ent 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 reaHzed that many of 

 the early forms that have been ascribed to the algae are relatively 

 imknown because of the poor preservation, and further examina- 

 tion of new specimens may mean that they will no longer have to be 

 classed as algae. For this reason it must be emphasized that there 

 are a number of doubtful forms from the lowest strata that can only 

 tentatively be assigned to the algae. 



Myxophyceae 



Among the unicellular forms a fossil which has been related to 

 the Chroococcaceae is recorded from the Ordovician. It is called 

 Gloeocapsomorpha and is a colonial form with cells that were ap- 

 parently enclosed in a jelly, and whilst it may have affinities with 

 Uving colonial forms it is usually placed in a group called the Proto- 

 phyceae. Another plant of Middle Cambrian age, Marpolia spissa 



Fig. 174 Marpolia spissa ( x 49-5). (After Walton.) 



294 



