The Relationship of th^ Tetracoralla to the Hexacoralla. i8i 



considerable range In size, it is now held that they are generically 

 distinct. 



Summary of "Paleozoic Hexacoralla." 

 From the above discussion it will be seen that a re-examination 

 of the "Paleozoic Hexacoralla" has led to the conclusion that 

 Calostylis, Heterophyllia, Hexaphyllia, and Battersbyia are genu- 

 ine Tetracoralla and that the genus Palaacis is a tabulate of the 

 family Leptoporidae. Compound forms like Axinura and Pachy- 

 phyllum, at first sight closely resembling Hexacoralla, prove 

 upon closer examination to have the bilateral symmetry and 

 arrangement of Tetracoralla. A further discussion of these and 

 similar genera is not attempted here, as their tetracoral nature 

 may be easily ratified by examining the many illustrations of 

 cross-sections which are available among the works of Rominger, 

 Lambe, Nicholson, and A. Vaughan. The final conclusion from 

 available data is that there are no known Paleozoic Hexacoralla. 



First Fossil Corals. 

 Mackenzia of the Middle Cambrian. 

 Excepting some forms of uncertain relationship comprising 

 the family Archaeocyathidse, of wide distribution in the older 

 Cambrian, nothing is known of stony corals earlier than the 

 Middle Ordovician (Chazy). On the other hand, a soft-bodied 

 form, Mackenzia, which may have some relation to the Anthozoa, 

 has been described by Walcott (1911) from the Middle Cambrian 

 of British Columbia. There are only two specimens known of 

 this remarkable fossil and great doubt exists as to its relation- 

 ship. It was at first placed among the Holothuroidea, while in 

 Zittel-Eastman (1913) it was suggested that it may be an 

 actinian closely related to Edwardsia. There is no calcareous 

 matter now present in the specimens, but the suggestion was 

 made by the author of the genus that "nearly all calcareous 

 matter was removed by solution in the mud deposit prior to its 

 consolidation and alteration into rock." The presence of an 

 Edwardsia-Xiko^ organism in the Middle Cambrian is a matter 

 of great significance because of the conclusion reached by 

 Bourne, Brown, and others that the original stock from which 

 the Anthozoa descended was one in which eight mesenteries 

 occurred in the adult. This conclusion is supported by the fact 



