1 88 W. I. Robinson, 



PLASTICITY OF TETRACORALLA IN THE LATE 

 PALEOZOIC. 



In the preceding discussion it has been shown that there are 

 none of the so-called Paleozoic Hexacoralla that may not more 

 definitely be placed in other sub-classes. It will next be pertinent 

 to investigate the variability of late Paleozoic Tetracoralla, since': 

 the facts of geological occurrence clearly point to the theory • 

 of direct descent of the Hexacoralla out of the former as the 

 more probable source. If the phylum has assumed, by the end 

 of the Paleozoic, a pattern from which only slight variations 

 occur, it may be possible to show that the variation is too limited 

 to give rise to new forms. If the stock is no longer vigorous, 

 how may it be expected to survive the increasingly desperate 

 trials of vitality to which it is destined because of the cold waters 

 of the Permian? On the other hand, if there is evidence of an 

 unstable variation, producing giants and monstrosities, we may 

 then look for the end of the phylum. 



The Columella as an Example of Variability. 

 There is one point of structure in which the late Paleozoic 

 corals quite generally depart from the patterns established in 

 earlier times. This is the presence of a columella, the most 

 characteristic evolutional tendency of the last great coral assem- 

 blage, the Lower Carboniferous coral reefs of western Europe. 

 Since the columella is nearly always present in some form in 

 Carboniferous corals, it may be taken as the measure of their 

 variability. In order to judge of this, we may, therefore, take 

 under consideration the morphological and physiological sig- 

 nificance of the columella. 



Morphological Significance. 

 Milne-Edwards defined a columella simply as a central 

 colimm. Four kinds were distinguished: 



1. False columella (pseudo-columella). Caused by a twisting 

 of the inner ends of the septa. 



2. Septal columella (columella septalis). A fasciculate 

 column composed of pali or similar structures. 



3. Parietal columella (columella parietalis). A spongy 

 central mass. 



