p. E. Raymond — History of Corals, etc. 345 



coralla in the early Mesozoic. No one of the former sur- 

 vived the Paleozoic ; none of the latter is represented by a 

 calcareous fossil in that system, Robinson having shown 

 that the supposed Paleozoic hexacorals really do not 

 belong to that group. ^ What caused the extinction of the 

 Tetracoralla, and whence did the Hexacoralla come f 

 Arguments that the latter were derived from the former 

 have been put forward repeatedly, but do not seem con- 

 vincing. That they had a common ancestry seems practi- 

 cally demonstrated by studies in ontogeny. 



It seems possible that the answer to these questions 

 may be connected with physical changes at the end of the 

 Paleozoic. The waters of the oceans were doubtless 

 greatly chilled at the time of the Permian glaciation, and 

 this may have caused the killing of the Tetracoralla. 

 That these animals had, like modern reef-building corals, 

 become adapted to life in warm seas is shown by the fact 

 that they are almost confined to limestone, and that prac- 

 tically all are of reef -building or reef-inhabiting nature. 

 Even the simple unbranched corals are found in great 

 numbers when found at all, and there is every indication 

 that before the end of the Paleozoic they had become fully 

 adapted to life in clear, shallow, warm waters. Many 

 tribes did not, of course, survive even to the end of the 

 Paleozoic, and in eastern North America perished dur- 

 ing the uplift which followed the deposition of the Middle 

 Devonian limestones. 



Tetracoralla appear first in the Middle Ordovician, and 

 although the earliest representatives now known are sim- 

 ple, it is very probable that further collecting will reveal 

 them in the Lower Ordovician and perhaps in somewhat 

 older formations. Mackenzia is a form which could have 

 been ancestral to either of the groups mentioned and it is 

 possible that they diverged as early as the Upper Cam- 

 bian. Edwardsia, with which Mackenzia is compara- 

 ble, is today an inhabitant of the sand, and does not 

 secrete a skeleton. Members of the family occur in shal- 

 low, cool waters about the British Isles. It might seem 

 that Mackenzia and its descendants remained, during the 

 Paleozoic, in the cooler waters along sandy and rocky 

 shores, and living thus in places where calcium carbonate 

 remains readily in solution, they did not get the lime 



* Trans. Conn. Acad. Arts and Sci., vol. 21, p. 145, 1917. 



