6 CALCAREOUS ALGAE OF THE MIDDLE EAST 



interpretation of a Palaeocene genus as possibly having been similar to the atypical living 

 Dasycladas in shedding gametes direct instead of by the usual dasyclad encystment, and in 

 the conclusion that the terminal umbrella-type fertile discs seen in the living Acetabularia may 

 be of different origin though of similar morphology in the fossil Clypeina. Also described is a 

 species of Cymopolia showing the actual transition from cladospore to choristospore organization. 

 The views of Julius Pia on the general course of dasyclad evolution are confirmed. No detailed 

 explanation of this evolution can be offered, but the decline of dasyclads in abundance and 

 importance throughout geological time, and their replacement by certain calcified Codiaceae 

 in this respect, are now considered to be due to the differing relation of calcification to repro- 

 ductive bodies in the two families, this itself probably due to their differing basic morphology. 



I. INTRODUCTION 



Present-day dasyclads are small single-cylindrical segmented or umbrella-shaped 

 green algae, calcined in varying degree, and occurring mostly in warm very shallow 

 waters in tropical and sub-tropical seas. The family is not a large one in number of 

 component genera, and many of these illustrate markedly the phenomenon of dis- 

 continuous distribution. Neomeris, for example, is largely divided in occurrence 

 between the East and West Indies. When the fossil record is examined, the relict 

 nature of the Recent survivors is seen at once. Ancestral forms range from the lower 

 Palaeozoic onwards, and show a variety of strange genera now extinct. At some 

 geological levels they occur in profusion over large areas, and are used as index 

 fossils. Although individual sizes are small, when compared with those of some 

 other marine algae, yet proportionally giant forms occur amongst the fossil dasyclads, 

 and the Lower Carboniferous Koninckopora has been estimated to have attained a 

 length of 50 cm. This phenomenon of former large size is also not uncommon with 

 relict groups. 



In the largely arid land-area now known as the Middle East a thick succession of 

 ancient sediments bears witness to the former occurrence there of the old Tethyan 

 Sea. From Upper Palaeozoic to Mid-Tertiary times conditions of repeated shallow, 

 warm-water, limy-bottomed shelf-areas afforded suitable environments for the 

 growth of calcareous algae, and although, palaeontologically speaking, collections are 

 rudimentary compared with those from Europe, yet examination of routine strati- 

 graphical samplings has shown a succession of algal floras. In these the largest single 

 group, taxonomically if not numerically, is the Dasycladaceae. Although they do 

 not form whole reef-like rock-masses as do the Corallinaceae, nor occasion a mono- 

 tonously distinctive rock-type as do the sand-like fragmentary remains of the 

 Chaetangiaceae, yet the dasyclads impress themselves upon the mind of the student 

 by the seemingly endless variety of structures, all based on a common plan. In their 

 evolution, as interpreted by the Viennese worker Julius Pia over a working lifetime 

 of thirty years, largely from European materials, there may be traced a progressive 

 elaboration of their verticils or whorls of side-branches, the reproductive structures 

 moving from within the stem-cell, first to within the side-branches, and then to 

 specialised outgrowths adjacent to the subsidiary branches. But superimposed on 

 this was a variability of calcification, as between one genus and another, and 

 apparently showing no progressive trend throughout geological time. Some, 

 heavily calcified, show hollow moulds of almost the whole set of branching structures 



