SUMMARY. 



On pages 159 to 160 two theories for the origin of the Hexa- 

 coralla are outhned. The first postulates a common ancestry 

 with the Tetracoralla and a lack of the skeleton-forming habit 

 in the Paleozoic Hexacoralla ; the second, a direct descent from 

 Paleozoic Tetracoralla to Mesozoic, Cenozoic, and Recent Plexa- 

 coralla. The data w^hich form the basis of this paper rather 

 strongly favor the second theory, although no single fact has been 

 found to actually conflict with either. The method by which this 

 general deduction was made may be summarized as follows : 



1. There are no known Hexacoralla in the Paleozoic. It is 

 believed that this fact strongly favors the second theory, for 

 otherwise it is necessary to add to the first theory the conception 

 that the stock which finally developed into the Hexacoralla con- 

 tinued throughout the Paleozoic as exclusively soft-bodied forms ; 

 and that the post-Paleozoic corals went through a comparable 

 series of changes which produced the same modifications in the 

 soft basal disk as those which were taking place in the skeleton- 

 secreting basal disk of the Tetracoralla. It thus would be neces- 

 sary to consider that a widespread tendency to an invagination 

 of the basal disk developed in the soft-bodied Paleozoic Hexa- 

 coralla, since a columella occurs so commonly in the Triassic 

 forms. 



2. The case of Turbinolia, a genus of living Hexacoralla, 

 whose early life history so closely parallels that of Tetracoralla, 

 suggests a very close relationship between the Tubinolidse and 

 Tetracoralla. 



3. The Cyathophyllidae and Zaphrentidae approached Mesozoic 

 time as strong stocks capable of important structural variation. 



4. A marked tendency among Carboniferous forms is the 

 widespread development of columellas. Even the conservative 

 genus Zaphrentis was subject to this change. The columella is 

 a far more prominent feature of the Hexacoralla than it is of 

 the Tetracoralla. A correlative tendency is toward an increase 

 in the number of septa and a consequent approach to radial 

 symmetry. 



5. Some change in structure or function in the coral polyp is 

 indicated by this development in the late Paleozoic. 



