Although the structure of Heliopora has been the subject of a careful memoir by the late Professor H. N. Moseley, a renewed examination of this interesting and isolated Alcyonarian has, for many reasons, seemed to me to be desirable. There are questions relating to the structure and formation of the hard parts which were not conclusively settled by Moseley’s paper, and the great increase of our knowledge of the anatomy of other Alcyonaria has rendered it necessary to overhaul all older work in the light of more arecent researches. Professor Moseley himself was anxious that the genus should be re-examined, and, some time before his death, he pressed me to commence work on some specimens which I had brought back with me from Diego Garcia. The material which I have used in the course of my work consisted of the above-mentioned spirit specimens from Diego Garcia, of the fragments of the specimens which served Professor Moseley for his original memoir, and a well preserved portion of a colony, which Dr. S. J. Hickson brought from Talisse, and gave to Professor Lankester, who kindly handed it over to me for examination. This specimen of Dr. Hickson’s was most useful, because the growing tips of the colony were preserved uninjured, and I was therefore able to make sections illustrating the structure at the point where most active growth takes place. Of dried specimens, I have had a large collection, including my own from Diego Garcia, Dr. Hickson’s from Celebes, and a number of specimens from the collection of the late Mr. George Brook, which Mrs. Brook has been kind enough to hand over to me, together with a rich and varied collection of other Anthozoa. Unfortunately the labels have become detached from most of the specimens, but those that remain show that the corals formed part of the collection made by Professor Haddon in Torres Straits.