CALCAREOUS ALGAE OF THE MIDDLE EAST 99 



no more direct relation to the ancestral non-calcified proto-Dasyclad than a naked 

 slug has to the unshelled proto-gastropod. If this is so, and if the present writer's 

 analysis of Pagodaporella is correct, then this condition had arisen by the beginning 

 of the Tertiary, but was and is uncommon. 



If the fossil sporangia are intrinsically uninteresting, their progressive evolutionary 

 movement within the dasyclad plant, from endospore to cladospore to choristospore, 

 is the most persistent trend in dasyclad evolution. In the genera studied, an endo- 

 spore genus (Atractyliopsis) occurs in the Permian, where it is accompanied by 

 cladospore genera. Cladospore genera predominate in the Mesozoic and survive to 

 the Palaeocene, where they are represented by such genera as Trinocladus and 

 Thyrsoporella, and Broeckella. Choristospore types of conventional pattern appear 

 in the Cretaceous (e.g. Cymopolia, which also shows a species intermediate in this 

 character) and are dominant today. In Recent genera with terminal reproductive 

 discs these structures are regarded (Fritsch 1935 : 397 ; Egerod 1952 : 341) as 

 homologous with reproductive structures borne laterally on the primary branches of 

 choristospore dasyclads e.g. Bornetella. If the extinct Clypeina is grouped with 

 Acetabularia and Acicularia, as suggested by various workers (see above p. 28), then 

 this specialized condition, expressed in Clypeina as serial fertile whorls rather than as 

 a single terminal disc, arose possibly in the Permian, certainly in the Triassic. That 

 is, it arose at the same time as, or later than, the change from endospore to cladospore 

 and before the evolution of the choristospore type proper so far as is known. This 

 suggests that the serial reproductive discs of Clypeina are homologous with the 

 swollen branches of cladospore genera, and that the genus (? Permian, Triassic- 

 Oligocene) is thus an earlier, different, achievement of this structure than the 

 Acetabulareae (? Jurassic, Cretaceous-Recent) show. It is noteworthy that the 

 radial tubules of Clypeina spp. do not show the calcified sporangial contents in fossils 

 as do those of Acicularia ; that is they correspond in this respect to the normal 

 swollen branches of cladospore genera, which only exceptionally display this condi- 

 tion e.g. Triploporella. Moreover, Clypeina became extinct in the Tertiary as did 

 other cladospore genera, while the living dasyclad flora, of choristospore genera, 

 includes Acetabularia and Acicularia. It seems likely, therefore, that the discs of 

 Clypeina represent an earlier development in time of the condition seen in Acetabu- 

 laria, morphologically similar but of dissimilar origin ; this is a not uncommon 

 phenomenon in evolution. Clypeina is therefore placed in the new tribe Clypeineae, 

 to include forms with reproductive discs considered to be of cladospore origin. 



From a strictly morphological point of view, the relegation of Actinoporella to the 

 synonymy of the earlier-proposed Clypeina, now known to contain a small minority 

 of stellate species somewhat like Actinoporella, is a logical proposal. However, the 

 earliest stellate Clypeina, the Valanginian C. marteli Emberger, is one of a group of 

 varied species appearing after the disappearance of the Upper Jurassic C. jurassica, 

 and presumably evolved from it at the time of the terminal Jurassic uplifts and 

 lagunal developments. Actinoporella itself co-existed in the Upper Jurassic with C. 

 jurassica. Pia's suggestion (1927 : 693) that Clypeina arose direct from Actino- 

 porella must await understanding of the Permian Clypeina, and possible new 



