separate and independent existence, and developing a simple 

 coral in its body-wall, while the larger number are social, being 

 composed of numerous living beings in organic union with each 

 other. In this latter case we meet with a compound coral, 

 certain parts of which are formed in the bodies of the various 

 members of the colony, while others are the secretion of the 

 soft bond of union which binds the colonists together. 



The more important points of the structure of simple and 

 compound Polyactinian corals were indicated, and it was pointed 

 out that in the same manner that a hexamerous symmetry was 

 usually distinguishable in the number of the tentacles and 

 mesenteries, the hard septa or vertical radiating portions of 

 the coral exhibited a corresponding arrangement. Notice, 

 however, was taken of the fact that in a considerable number 

 of extinct forms of Polyactinia (those designated Tetracoralla 

 or Rugosa) the septa formed multiples of four, and that in some 

 of the species the coral exhibited the singular peculiarity of a 

 calcareous plate or operculum, with which its calice was closed. 



The Octactinian corals were next examined, and the various 

 modifications examined. Beginning with the very rudimentary 

 peculiar skeleton of Alcyonium, the various forms of skeleton 

 possessed by the Pennatulidae, Gorgom'dae, Tubiporidae, and 

 Helioporidae were exhibited and described ; and reference was 

 made to the important discoveries of Professor Mosely in con- 

 nection with the last family. 



The first part of the lecture was brought to a conclusion 

 with a brief notice of the Hydrozoan corals, of which two 

 recent families, Milleporidae and Stylasteridae, are now known ; 

 and it was stated as not unlikely that the problematical extinct 

 Stratoporidae belonged to the same class. 



The second portion of the lecture was occupied with a resume 

 of some of the more important points connected with the 

 structure, the geographical and bathymetrical distribution, and 

 the growth of coral reefs ; a sketch of Mr. Darwin's hitherto 

 generally accepted subsidence theory being given, and certain 

 objections to it recently advanced by Mr. Murray, of the 

 Challenger expedition, shortly noticed. 



