CALCAREOUS ALGAE OF THE MIDDLE EAST n 



which constriction of the individual branches and branchlets occurs without division 

 at these constrictions e.g. Palaeodasycladus. 



The reproductive structures, usually termed sporangial cavities in the fossil 

 literature, are often conspicuous features of the branch-detail, and are of great value 

 in classification. They may be completely encased in the calcareous coating where 

 this is well-developed, and indeed, in most similar living forms, gametes are only set 

 free from resting cysts on the eventual post-mortem break-up of the calcareous 

 structures. General classification of the condition seen in fossils is again tripartite 

 and due to Pia. The endospore type is presumed to have had reproductive elements 

 within the stem-cell. This is characteristic of the Palaeozoic and only such forms as 

 the doubtful Atradyliopsis already mentioned and a species of Diplopora show any 

 direct structural evidence of this condition. In the cladospore type, dominant 

 throughout most of the Mesozoic, the reproductive cavities are considered to have 

 been within swollen lateral branches. Finally, in the choristospore type the repro- 

 ductive elements are located in special outgrowths, usually well-calcified, from the 

 lateral branches. Commonly they are attached at the division of primaries into 

 secondaries (e.g. Cymopolia) but various other positions characterise other genera. 

 This type ranges from Cretaceous to Recent. Rezak (1959a) has drawn attention to 

 the parallel between Pia's view of the migration of reproductive elements during 

 phylogeny from stem-cell to lateral sporangia, and Egerod's summary (1952) of the 

 migration during ontogeny of the dividing nucleus from the holdfast of the vegetative 

 thallus into the gametangial rays of the reproductive thallus. Recent studies of the 

 genetic mechanisms involved in this latter phenomenon in the living Acetabularia 

 (Brachet 1965), do not invalidate this comparison. Summary of the chromosome 

 mechanisms in the Dasycladaceae (Puiseux-Dao 1966) emphasizes their distinctive- 

 ness amongst green algae. 



There exists a minority of dasyclads which depart from the usual pattern of a 

 vertical calcareous cylinder described above. 



Vermiporella and the somewhat doubtful Psendovermiporella from the Palaeozoic 

 are recumbent-irregular in form, indicating a presumed creeping thallus in life. 

 The former branches, and the latter shows a peculiar double calcareous structure in 

 part of the thallus. This is dealt with fully in the systematic descriptions below. 



In the Palaeozoic Mizzia and certain later genera the stem-cell occurred as con- 

 secutive bead-like structures, almost always dissociated as found fossil. Certain 

 Tertiary dasyclads (e.g. Larvaria) were of normal tubular pattern but the verticils 

 came apart after death and usually occur fossil as separate ring-like structures. 



Another and commoner growth form is where the plant appears with a thin, 

 lightly-calcified stem supporting a specialized flower-like disc or series of discs, 

 usually found separate as fossils, in which the radial segments or " petals " are 

 calcified structures containing the reproductive elements in life (e.g. Clypeina). In 

 Acicularia these segments themselves came apart after death and occur as separate 

 microfossils, of varying form, known as spicules. Not all such spicules, however, 

 have this origin : some, such as the Tertiary Carpenterella, are believed to be 

 dissociated structures from the interiors of dasyclads of modified cylindrical form. 



