i6o IV. I. Robinson, 



secreting Tetracoralla and a soft-bodied group which later gave 

 rise to the Hexacoralla. Moreover, they assume that the evolu- 

 tion in the soft-bodied forms so closely paralleled that of the 

 Tetracoralla that when the former finally adopted the skeleton- 

 forming habit they had developed the same specializations of 

 the basal disk. It is difficult to suppose, for instance, that the 

 soft-bodied hexacorals of the late Paleozoic acquired a basal 

 invagination as the Tetracoralla did, and this is necessary, under 

 the theory of common ancestry, to account for the widespread 

 occurrence of the columella in the Mesozoic. 



THEORY OF DIRECT DESCENT. 



The second theory is that the Hexacoralla descended from the 

 Tetracoralla in late Paleozoic or, what is more probable, in early 

 Mesozoic time. A comparative study of early ontogenetic stages 

 should be valuable, but the chief test of this theory is whether 

 or not a sufficiently close relationship can be established between 

 the Hexacoralla of the Middle Trias and the late Paleozoic 

 corals. The greatest difficulty in applying this method is a lack 

 of data. The record of corals in the Permian is, as a rule, 

 meagre, though in India there seem to be many of them, and 

 a few are known from the Australian "Carbopermian" ; but 

 there is as yet no knowledge of any corals from the Lower 

 Triassic. 



The first consideration in comparing these two theories is, 

 whether or not there are any Paleozoic forms which can con- 

 fidently be referred to Hexacoralla. If such forms occur, then 

 the second theory must be modified or abandoned. 



Two Paleozoic genera, Palceacis and Calostylis, have been 

 repeatedly, though always with some opposition, designated as 

 Hexacoralla. There are, however, very good reasons for saying 

 that neither of the genera has any such close relationship with 

 modern corals, as will be seen by the following analysis of these 

 perplexing forms. 



