1 66 THE GREAT BARRIER REEF. 



its allies, previously described, in that the corallites or polyp centres, in place of being 

 crowded together so thickly that their divisional or septal walls are closely united, are more or 

 less widely separated ; the intervening spaces being filled in, after the manner of Galaxea 

 previously described, with a separate calcareous deposit, technically termed the " cosnencyma." 

 As a result of the absence of the lateral pressure that induces the polygonal contour of the 

 corallites of Goniastraea and its allies, those of the group now under consideration are all, more 

 or less, symmetrically circular. In certain of the genera of this group, such as Plesiastrsea, 

 Cyphastraea, and Heliastraea, whose habit it is to expand in the full sunlight, the colours of both the 

 polyps and the intervening investing membrane, or " coenosarc," are conspicuously brilliant. Thus, 

 in the form apparently identical with Plesiastraa vcrsipora, illustrated by Plate VI., Fig. 2, of the 

 coloured series, the common investing membrane is a bright peacock-green, and the oral centres of 

 the polyps are white, and the tentacles dark brown. In a second species, allied to P. Pcronii, 

 Figs. I and 5 of the same plate, the ground colour is lilac, and the pol3'ps light grey-green with 

 bright lilac-tipped tentacles; or, again, the polyps may be a delicate lilac, and the tentacles white 

 tipped. In all the species just enumerated the polyp-cells or corallites are small, not exceeding 

 a quarter of an inch in diameter. The polyps are at the same time capable of extension to the 

 distance of an inch or more beyond their cell-orifices ; and in this respect, together with their 

 possession of twenty-four subulate tentacles, they closelj' resemble those of the perforate coral 

 section hereafter referred to, under the genera Alveopora and Rhodoraea. 



The coralla constructed by the types last named are, as compared with those of the Goni- 

 astraeae and Meandrinas, rarely of large dimensions, and they may assume either an encrusting 

 hemispherical or an irregularly lobate contour. The area of distribution of Heliastraea and its 

 allies is co-extensive with that of the Barrier district. The dead coralla of a species of Cyphastraea 

 have been collected by the author as far south as the shores of Moreton Bay. 



A genus belonging to the same tribe of the Astraeaceae, viz., Favia, is still living in the 

 last-named locality, and represents one of the few survivors of a number of species that until 

 within a comparatively recent date flourished in this southern, extra-tropical, region. Both the 

 coralla and the individual corallites of the genus Favia attain to a large size, the former being 

 often represented by hemispheres, or sub-spherical agglomerations, two feet or more in diameter, 

 while the corallites may be from half to three-quarters of an inch in width. Like those of 

 Cyphastraea and Heliastraea, the corallites of Favia, in their typical condition, are sub-circular, 

 and separated from one another by a greater or lesser amount of intervening calcareous matter, 

 or coenenchyma. It was observed of the species common to Moreton Bay, which very nearly 

 resembles the Favia Ehrenbergi of Klunzinger, that a very considerable amount of structural 

 variation may occur in a single corallum. Specimens were obtained, in point of fact, in which, 

 while in one portion of the corallum the corallites exhibited their normal distinctly separate 

 circular contour, in other parts they were crowded together without any mtervening spaces, and 



