ioo CALCAREOUS ALGAE OF THE MIDDLE EAST 



Mesozoic records. His interpretation of Adinoporella as similar to the Triassic 

 Oligoporella but differently calcined is followed here : Adinoporella is regarded as a 

 valid genus and left in the Diploporeae. 



These problems apart, the evolutionary change in position of dasyclad reproductive 

 structures is the most significant feature of their long history. It has been written 

 (Elliott 1962) "we do not clearly know the special advantage of choristosporic 

 structures, whether a direct one in shedding reproductive elements more easily, or 

 an indirect one in their being produced more freely and lavishly with no greater or 

 even less strain on the metabolism of the plant, but although not properly understood 

 it is a main trend in dasyclad evolution ". 



Although no precise elucidation of this point can be offered here, the trend itself, 

 clearly indicated by Pia (1920), has been confirmed by subsequent work including the 

 present study. Whatever its significance, it has not saved the Dasycladaceae from 

 decline to a very subordinate position indeed in modern marine floras. Although 

 growths of living dasyclads sometimes form extensive patches or thickets on the 

 sea-floor, I know of no existing deposit in which a great thickness of sediment 

 consisting principally of their calcareous tubes is being accumulated. A prolonged 

 and persistent dense growth of dasyclads to build up considerable thicknesses of 

 dasyclad limestones as are known from the Alpine Trias, e.g. the Essino Limestone 

 of northern Italy, is apparently a thing of the past. Pia (1920 : 187) attributed this 

 decline of the dasyclads principally to the development and spread of the articulated 

 Corallinaceae, with different cellular organization, from the Cretaceous onwards. 

 Much of this spread was into environments for which the dasyclads were not suited, 

 by reason either of the mechanical effects of water movement or of lower tempera- 

 tures. The appropriate modern ecological analogue comes from a related family of 

 green algae, the Codiaceae, where Halinteda occurs in dense growths, notably in the 

 lagoons of atolls and other back-reef environments once largely colonized by dasy- 

 clads. Calcified segments from generation after generation of plants pack down to 

 form true ##^'m^a-limestones, well evidenced by borings into the reefs of these 



Fig. 15. Diagrammatic vertical sections of comparable portions of the dasyclad Cymopolia 

 (left) and the codiacean Halimeda (right). Plant tissue stippled, calcareous structure 

 black. To show the encased dasyclad sporangia and free codiacean reproductive growth. 

 Greatly enlarged. 



