From the " Proceedings of the Zoological Society of London," 



April 5, 1S98. 



ON THE SPECIES OF THE GENUS MILLEPORA : A PRE- 

 LIMINARY COMMUNICATION. 



By Sydney J. Hickson, M.A., D.Sc, F.R.S., F.Z.S. 



The phylum Coelentera presents us with many families and orders 

 of animals in which our knowledge of the characters which can be 

 satisfactorily used for the purpose of systematic classification is singu- 

 larly deficient. In the Madreporaria, the Gorgonacea, and the Mille- 

 poridse the form of growth of the colony, the colour, and the structure 

 of the hard skeletal parts are the only characters which have been used 

 for the diagnosis of genera and species. In many cases it is probable 

 that the diagnosis afforded by these characters should be considered to 

 be satisfactory, but as the number of specimens in our museums 

 increases it becomes more evident that in others no satisfactory classi- 

 fication can be framed until we have a thorough knowledge of the 

 anatomy of the polyps which construct these skeletons and of the canal- 

 systems which bind them together into colonies. 



In some genera of Madreporaria, for example, of which the skeletal 

 characters only are known, a long series of intermediate stages can be 

 found between the type specimens of the different species, and every 

 new collection of specimens that is examined increases the difficulty of 

 deciding whether a particular intermediate form belongs properly to one 

 species or another. Moreover, in this same group the outlying species 

 of one genus resemble the outlying species of another so closely that it 

 is often a matter of great difficulty to determine, on our present system,. 

 to what genus a particular specimen belongs. 



Nearly every important systematic work on these Ccelenterates 

 contains some remarks about the difficulty of determining species, and 

 examples are quoted of series of intermediate forms connecting closely 

 allied species. If it were possible to frame some general rule for the 



