b L. HILLIS-COLINVAUX 



were formerly classified in the order Siphonales. Hence they often are 

 described as siphonaceous. 



Cross walls in the filaments have not been observed to form regularly 

 at any stage in the life-history of Halimeda. In this way, and in its 

 regular multinucleate condition, this genus differs from Acetabularia, 

 another well-known calcareous green alga. 



.|ini|inf|?ifijfiir|i!iiitur| 



12 3 



Fig. 4. H. cylindracea, from the lagoon of Enewetak Atoll. The specimen has been 

 decalcified to show the extensiveness of the mass of holdfast filaments which are 

 normally obscured by adhering particles of substrate. 



