FOSSIL FORMS 



269 



and has been tentatively related to Halimeda. It is about ten 

 millimetres long and is composed of branched tubular cells without 

 any cross walls, the cells being embedded in a calcareous matrix. 

 Ovulites, a genus which occurs up to the Eocene, differs consider- 

 ably from those previously described: the species are little egg or 

 club-like chalk bodies beset with fine pores and with a large opening 

 at what was either the base or apex. It has been suggested that 



A B 



Fig. 170. Codiaceae. A, Palaeoporella 

 variabilis ( x 12). B, Boueina Hochstetteri. 

 (After Hirmer.) 



Fig. 171. Dasycladaceae. 

 RJiabdoporella pachyderma 

 ( >< 135)- (After Hirmer.) 



perhaps they represent siphonaceous plants in which the apical 

 tuft of threads has been lost. 



Dasycladaceae 



This is the best know^n group and contains a very large number 

 of the fossil algae. It reached its maximum development and 

 abundance in Carboniferous and Triassic times, and in those days 

 was far more important than its present living representatives. The 

 various forms are all based on a type of construction which can be 

 sufficiently explained by descriptions of a few of the more repre- 

 sentative types. 



RJiabdoporella (fig. 171) seems to be one of the most primitive 

 genera as it is represented by a purely cylindrical shell that is 



