GENERA OF CIEEJETID.E AND MONTICULIPORID.E. 273 



is probable that a considerable number of the so-called en- 

 crusting MoiiticidiporcB will prove, upon adequate examination, 

 to be truly of a Polyzoan nature. Be this as it may, it is quite 

 certain that the mere form of the corallum, though affording a 

 useful guide to the collector, is usually of no value whatever in 

 determining the structure and affinities of a given specimen of 

 Alonticulipora. As an illustration of this fact, I may mention 

 that among the corals which, from their general form and 

 superficial characters, would unhesitatingly be placed under the 

 well-known species M. petropolitana, Pand., I find at least 

 three well-marked types to be included, which differ so widely 

 from one another in minute structure, that they might well be 

 regarded as at least distinct sub-genera. At the same time, 

 certain species, and especially those which have a laminar or 

 Irondescent corallum, are very constant in their mode of 

 growth, so that in these cases the form of the corallum really is 

 of value in the determination of species ; while the ramose 

 species, however variable, never appear to form crusts on 

 foreign bodies, as some of the massive species occasionally do. 

 The corallites in Montiailipora may be distinctly prismatic or 

 polygonal, or they may be rounded and nearly cylindrical, but 

 in either case they are always in close contact, and they never 

 really exhibit the condition of parts characteristic of Chcetetes 

 proper, in which, as has been shown, the walls of contiguous 

 corallites are so completely amalgamated that the original lines 

 of demarcation between neighbouring tubes cannot be in any 

 way detected. In some cases {e.g., in the typical M. petropoli- 

 tana, Pand.) the walls are so thin that the partitions between 

 the visceral chambers of contiguous corallites appear to be 

 absolutely indivisible, and present themselves in thin sections 

 merely as delicate dark lines (fig. 35, a). This is, however, a 

 state of parts very different to what occurs in Chcstdcs proper 

 (fig. 35, d), and is much more nearly comparable to what we 

 observe in many species of Favositcs. In other cases, the 

 condition of things is very like that observable in Favositcs 

 generally, in which the walls of contiguous tubes are distinct, 



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