200 THE GREAT BARKIER REEF. 



species of Gorgonia collected on the Palm Island reefs, apparently identical with the Plcxanra 

 salicoriioidcs of Milne Edwards, is, as a matter of fact, the only type the author found growing, in 

 a position accessible by wading, throughout the reefs explored. The dead coralla of several 

 forms of more than ordinary interest are, at the same time, washed up in some quantity on 

 the coral beaches, and consequently contribute a small quota towards the solid reef-formation. 

 Other conspicuous forms are not unfrequently brought in by the pearl-shell divers, from the 

 fishing grounds in Torres Strait. These arborescent Alcyonaria exhibit several distinct 

 growth-formations. In the group represented b}' the typical genus Gorgonia, the corallum takes 

 the form of a tree- or bush-like structure having a central, more or less flexible, horny axis, 

 which is enclosed within a bark-like cortex. This external cortex, which is alone visible in the 

 living corallum, represents the associated polyps with an intervening matrix bearing aggregated 

 spicules, analogous to that which alone builds up the leathery polypary of the typical Alcyonidas. 

 The surface of this outer cortex is found, on near examination, to be thickly perforated with 

 minute holes ; from each of these, in the living corallum, an eight- pinnately-tentacled 

 polyp expands or contracts at will, as obtains among the simpler leathery Alcyonidas above 

 referred to. A characteristic illustration of this particular group is furnished by Ctcnocella 

 pectinata, represented by Plate XL, Fig. 3, of the chromo-lithographic series, in which 

 the coralla present a peculiar comb-like form, all the secondary branches being developed in 

 the same plane and on the same side of the main rachis. The colour of the outer, spiculiferous, 

 cortex in this species exhibits a wide range of variation, being in some examples a bright 

 brick-red and in others creamy-white, while every gradation of tint between the two may be 

 met with. The expanded polyps, as is the case with almost all members of the Gorgoniaceas, are 

 colourless, and nearly transparent. A nearly allied, more normal, representative of the same 

 group, Corgonia australicnsis, also commonly obtained by the pearl-shell divers in Torres Strait, 

 is delineated by Fig. 4 of the chromo plate above referred to. 



Among the species that are not uncommonly washed up on the coral beaches of certain 

 of the Capricorn group of islands, one form, his hippuris, is remarkable for the fact that 

 its corallum is built up of two distinct elements, that are distributed in alternate joints of 

 variable lengths. The more massive of these is white and chiefly calcareous, being com- 

 posed of consolidated spicules ; while the other is black, and composed almost exclusively of 

 the dense horny material that constitutes the entire axial corallum in the typical Gorgonidae. 

 The cortex and polyps in this singular jointed coral are indistinguishable from those of the 

 species last described. An illustration of this interesting type is given in Figs, i, ia, ib 

 of Chromo plate No. XI. Another noteworthy species of the Gorgoniaceous tribe is repre- 

 sented by Melilodcs ochracca, a robust arborescent type, whose erect, branching corallum may 

 attain a height of two or three feet, and its main trunk a thickness of three or four inches. 

 The corallum or "sclerobase" in this instance more nearly resembles the calcareous internodes 



