248 PROF. S. J. HICKSON ON THE [-A-pr. 5, 



which may be used for distiuguishiug species iu Cujleiiterates, it 

 may be well to describe briefly the general results of my obser- 

 vations on the genus Millej^ora. This genus stands quite by itself 

 among living corals. No one genus of the other Hydrocorallines 

 can be confused with it, both the living tissues and the hard 

 skeletal parts being perfectly distinct. It is widely distributed 

 through the tropical seas, occurring in the Red Sea, Indian Ocean, 

 Malay Archipelago, Tropical Australian waters. Pacific Ocean, and 

 in the seas of tlie AVest Indies. It is essentially a shallow-water 

 genus, living iu abundance in most of the coral-reefs, and not 

 occurring in greater depths than 15 fathoms. 



The form varies immensely. It may be broadly lamellate or 

 densely branched, or anastomosing, or it may form thin incrusting 

 plates on dead corals. In all large collections of Millepores series 

 of intermediate forms may be found between all the most pro- 

 minent types. 



The clifficulty of defining and describing the species of this 

 genus has been commented upon by several authors. Dana, for 

 example, says " There is much difficulty in characterizing the 

 IMillepores on account of the variations of form a species under- 

 goes and the absence of any good distinctions in the cells. The 

 branched species are often lamellate at the base, owing to the 

 coalescence of the branches, and the lamellate species as well as 

 the branched sometimes occur as simple incrustations." My own 

 investigations confii^m and amplify Dana's statements on this 

 point. 



Notwithstanding these difficulties a large number of species of 

 the genus have been described. In the writings of the older 

 naturalists many species were described which have since been 

 relegated to other classes of the animal kingdom, and in palseonto- 

 logical literature we find many species of fossil corals referred to 

 the genus on erroneous or very unsatisfactory grounds. 



Apart from all these, which may be left out of consideration in 

 this paper, no less than 39 species of the genus Millejiora have 

 been described. 



The characters which have been used for determining these 

 species are: — (1) The form of the corallum. (2) The size of the 

 pores. (3) The degree of isolation of the cycles. (4) The pre- 

 sence or absence of ampullse. (5) The texture of the surface of 

 the corallum. 



(1) The Form of the Corallum. — This feature is even more 

 unsatisfactory than I anticipated at the beginning of my inves- 

 tigation. In the first place, attention has been called by Dana, 

 Duchassaing and Michelotti, and others to the fact that MiUepora 

 grows in an incrusting manner on many objects, and thereby 

 assumes the form of the object on which it grows. It is quite 

 easy to distinguish such forms as incrusting forms when they 

 have only partially covered such objects as the horny axis of a 

 Qorgonia, a glass bottle, or an anchor ; but in many cases the 

 object is so completely overgrown by Millepore and other marine 



