30 THE GREAT BARRIER REEF. 



the immediate central foreground, and the greater portion of the posterior area of the reef, differs 

 from the preceding type in the more profusely divided, digitiform, prolongations of the peripheral 

 edges of its polj^paries. Its customary line tints are varying shades of lilac ; it would appear to 

 be identical with the Alcyoniuin JJcxibilc of Dana. A third, but as yet undetermined, species of the 

 same genus forms two slightly separated patches to the extreme left of the centre and the back- 

 ground of this reef-view. It presents, in life, the appearance of closely crumpled leather of a 

 golden-brown hue, and is apparently identical with the species illustrated by the lower figure 

 of the following plate, and there associated with the title of Alcyoniinn iiiiirale. 



The so-called " Blue coral," Heiiopora caenilca, of which a fine corallum is stationed in the 

 centre of the near foreground, while a younger growth may be detected farther to the right, well 

 merits the place of honour that it adventitiously occupies. The high interest attached to this 

 species, as the only known living representative of the Alcyonarian order that fabricates a solid 

 calcareous corallum, together with all essential data concerning its structure and affinities, is fully 

 dealt with in the chapter that is devoted specially to coral organisms. In the Organ-pipe coral, 

 Tubipora, which constituted the subject of the preceding plate, the corallum, while rigid and 

 calcareous, is formed out of thin, loosely aggregated tubuli, while in the present form, Heiiopora, 

 it is as dense and ponderous as that of a Porites. The popular name associated with this species 

 is derived from the deep indigo-blue colour of its internal substance as exhibited in sections 

 when snapped asunder. The exterior surface is usually of an unattractive light slate, or bluish- 

 grey hue ; but in some instances the distal growing edges are light yellow. This lighter colour 

 is very distinctly indicated in the luxuriantly growing corallum included in this reef-view, while 

 an illustration of the natural lines of the corallum, and the aspect of the constituent polyps, 

 is embodied in Figs, i and 2 of Plate X. of the chromo-lithographic ■series. 



A very characteristic representative of the same Alcyonarian class remains to be noticed 

 in association with this reef-view. The species referred to forms incrusting masses of sub- 

 cylindrical or clavate polyps of semi-cartilaginous consistence that are closely united to each 

 other by a reticulate, or intricately interlacing, basal rhizome. This species, which appears to be 

 closely allied to Clavularia viridis, will be found represented by a large reticulated mass to the ex- 

 treme right of the nearest foreground, other, less distinct, colony-stocks being recognisable at farther 

 distances to the rear in the same straight line. In the condition illustrated by this reef-view, 

 all the polyps are necessarily contracted. In their submerged condition, when fully expanded 

 and associated together in dense clusters, they present an exceedingly beautiful spectacle. The 

 individual polyps, as shown in Fig. 17 of the Chromo plate No. X., are of considerable size, and 

 each of the eight tentacles is profusely clothed with brilliant golden-green pinnules. As the 

 polyps are massed together in such numbers that their individuality is indistinguishable, the ex- 

 panded colony-stocks present an aspect that may be most appositely compared with luxuriant tufts 

 of some very brilliant h3'pnoid moss. This Clavularia appears to be confined to the equatorial 



