reproduction; evolution 



297 



maximum abundance in the late Cretaceous. Nearly 200 genera 

 have been described and 120 of these are still living. Fossil diatom 

 deposits are known as Kieselguhr and are valuable economically. 



Chlorophyceae 



Codiaceae 



Boueina (cf. Fig. 177) is an unbranched form from the Lower 

 Cretaceous, whilst Palaeoporella (Fig. 177), which was composed of 

 hollow cyHnders of funnel-shaped bodies with slender forked 

 branches, the whole being 2-14 mm. long, comes from the Lower 

 Silurian. Dimorphosiphon, from the Ordovician, is generally re- 

 garded as the oldest known member of the Codiaceae and has been 

 tentatively related to Halimeda. It was about 10 mm. long and com- 

 posed 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 considerably from those previously 

 described: the species are Httle 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 perhaps they represent siphon- 

 aceous plants in which the apical tuft of threads has been lost. 



B 



Fig. 177 Codiaceae. A, Palaeoporella variabilis ( x 12). B, Boueina 



hochstetteri. (After Hirmer.) 



Dasycladaceae 



This is the best known group and contains a very large number 

 of the fossil algae. It reached its maximum development and abun- 

 dance in Carboniferous and Triassic times, and in those days was 



u 



