CALCAREOUS ALGAE OF THE MIDDLE EAST 101 



islands, and by many records of Halimeda-limestones in the Tertiary of tropical and 

 subtropical latitudes. Since Halimeda is quite heavily calcified, it is not calcification 

 itself which has impeded the spread of dasyclads amongst more numerous non- 

 calcified green algae in suitable environments. The explanation probably lies in the 

 reproductive mechanism : Halimeda sheds abundant gametes freely from special 

 deciduous non-calcified outgrowths from the segments. In almost all dasyclads the 

 release of the resting cysts depends on eventual break-up of the calcareous structures. 

 The reproductive bodies of Halimeda are almost unknown in a fossil state, a unique 

 occurrence being recorded by Pfender (1940 : 245), whereas those of dasyclads 

 survive unbroken in a majority of the specimens found fossil. There is some evidence 

 which has been interpreted as indicating that Halimeda was of hybrid codiacean 

 origin in the late Cretaceous (Elliott 1965b) ; certainly its spread and abundance in 

 the Tertiary and at the present day is remarkable. For all their morphological 

 elaboration, the dasyclads did not overcome the initial handicap inherent in the 

 calcification of the reproductive bodies ; this in turn springs indirectly from the basic 

 morphology of laterally whorled stem-cell, the Codiaceae by contrast being formed 

 of richly-branched, agglomerated threads with largely marginal calcification between 

 these and not encasing them, at any rate in the actively growing segments. There is 

 some similarity with marine invertebrate evolution, where, amongst the brachiopods, 

 progressive evolution of very complicated calcareous supports for the fleshy lopho- 

 phore did not compensate for the basic inferiority of this organ when compared with 

 the functionally comparable gill of the lamellibranch mollusca. 



It is difficult, and perhaps impossible, to effect a quantitative consideration of the 

 factors considered above, but they are considered to be in some measure responsible 

 for the " relict group " nature of existing dasyclads, and for their general subordinate 

 position amongst modern marine algal floras. 



VIII. REFERENCES 



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