An attempt has been made in this work to place the massive species of the [coral] genus on a sound basis. A number of new species have been described, and type or topotype material of previously described species has been investigated. In cases where forms have been referred to extra-Australian species the types or topotypes of these have been examined as far as possible. This was done by the writer for many of the European species, but it was not possible to investigate many of the American species in this manner, and, as little American material has been described from thin sections, comparison with American species has been difficult; some of the forms described as new by the writer may possibly prove to be referable to American species, when the latter have been investigated by means of thin sections.
Of those forms which have been ascribed to extra-Australian species, the description given is that of the Australian material.
The writer is indebted for much advice as to methods and other matters. to Dr. Stanley Smith, of the University of Bristol, and to Dr. G. L. Elles, of Cambridge. He wishes to express his thanks to Dr. R. S. Allan for allowing him the use of his large Silurian collection, and to the authorities of the Australian Museum, Sydney, and the National Museum, Melbourne, for the loan of material; to Professor H. C. Richards and Drs. W. H. Bryan and F. W. Whitehouse for the loan of specimens from the University of Queensland collection; to Dr. W. D. Lang for the loan of specimens from the British Museum; and to Dr. G. L. Elles for reading the manuscript and many useful suggestions.