508 MR. H. M. BEENAED ON THE AEriNITIES OF 



thickenings of the septa themselves. My attention was first called 

 to these dissepimental thecae by studying the growing edge of an 

 Astrseid colony (not yet classified). The thick rouud-topped 

 theca was rising between the young septa, and was so obviously 

 dissepimental that when I applied a pin it punctured like thin 

 paper. On looking over other Astrseids I found the same 

 dissepiments, in all stages of obscuration however : fusions of the 

 septa inter se had taken place, but closer examination showed 

 that in many cases, even when the synapticular theca was 

 developed, from the uppermost wall of the theca dissepimental 

 iloors sloped away both inwards and outw^ards ; in some cases 

 again only the external, in others only the internal dissepiments 

 were now recognizable. It has of course long been known that 

 the rich development of dissepiments was a characteristic of the 

 Astreeids, and Martin Duncan* even claimed that the "true 

 theca" of Cceloria and of Leptoria had been "replaced" by 

 epithecal nodules. Further, this very rich formation of dissepi- 

 ments mounting to the level of, if not actually forming, the 

 theca doubtless suggested to Milne-Edwards and Haime their 

 great section Madreporaria aporosa, for thecse so formed, or at 

 least covered, would necessarily be imperforate. 



I must here, however, perforce content myself with merely 

 throwing out this suggestion. Its elaboration will require an 

 extended study and sorting out of the different genera now 

 commonly classed together as Astrseidae. Tor in addition to the 

 dissepimental theca, there appears to be another common among 

 " Astrseidae," the origin of which will be discussed later on. 



So much has been written about the synapticular theca — the 

 " pseudotheca " of von Heider — that I need say little about it. 

 It can hardly be looked upon as a structure sui generis, being 

 essentially septal in origin, and only a further complication of 

 the original ectodermal infolding, and developed apparently for 

 the mutual support of the tall septal plates (c/, figs. 8 & 9). The 

 synapticular theca would obviously be much stronger than the 

 dissepimental, and more readily obtained by the multiplication 

 of the septa. 



Before dismissing this subject of thecal formation it is well to 



ask whether there may not be another, namely, a true circular 



fold of the basal ectoderm of equal value with the septa as a direct 



infolding of the external body covering, and interrupted by the 



* Journ. Linn. Soc, Zool. xvii. 



