1 86 THE GREAT BARRIER REEF. 



(recorded with the object of ascertaining the rate of growth of the associated corals by- 

 hereditary observation), is likewise composed of the same species of Porites. 



In addition to the massive-growing representative of the species just described, there are 

 many other varieties of Porites, indigenous to the Barrier Reef. Some of these exhibit, on a 

 smaller scale, a similarly solid habit of growth ; while others construct coralla which may be 

 irregularly lobate, digitate, or even arbuscular. The coralla of the two last-named growth- 

 types considerably resemble those of the imperforate genus Stylopora ; and it is noteworthy 

 that the living polj-ps of all the representatives of the genus Porites examined b}' the author, agree 

 structurally with those of Stylopora, their tentacles being twelve, and distinctly knobbed or 

 capitate at their extremities. This fact is of interest, as demonstrating that the classification 

 of the coral class on the basis of the structure of the corallum alone is to a large extent 

 empirical, and fails to bring into close alliance many forms that are naturally related to one 

 another by virtue of the similarity of their soft parts. 



The colours of the coralla and the associated pol3'ps are, in the genus Porites, neither as 

 varied nor as brilliant as are those of the two genera previously described. A light ochre, 

 dark and golden or mustard yellow, and brown, are the prevailing hues among the arborescent 

 types. The surface of the corallum in the massive species, however, is often a delicate pink, 

 a light or bright lilac, or (more rarely) pale yellow. The abnormally large massive block 

 represented in the Palm Island reef-view, Plate VI., was of a pinkish-lilac hue, as were 

 many coralla belonging to the same species in that district. The living polyps, as above 

 mentioned, possess twelve tentacles, the extremities of which are distinctl}' inflated. While the 

 oral disks and bases of the tentacles usually correspond in colour with the corallum, the inflated 

 tops of the tentacles are most frequently greenish-grey. Coloured representations of three typical 

 forms of the genus Porites, with enlarged delineations of their associated polyps, are included 

 in Chromo plate No. VIII. 



The genus of perforate corals that invites attention next to Porites is that of Alveopora. In 

 this generic group the component cells of the corallum, or corallites, are of larger size than 

 those of Madrepora, Montipora, and Porites, resembling somewhat, in their close approximation 

 to one another, the cells of a honeycomb. In texture, these corallites are of exquisite tenuity, 

 as though constructed of delicate lacework ; while the septal elements, which in the majority of 

 corals are represented by distinct plates or lamellae, are in this genus replaced by simple spinous 

 processes. The contour of the coralla in the genus Alveopora is usually more or less rounded or 

 deeply lobate. In one variety, however, allied to Alveopora clavaria, dredged by the writer in the 

 neighbourhood of the Lark Passage, the corallum exhibits a sub-dendroid type of growth. In the 

 sub-globose or irregularly lobate variety of Alveopora viridis, that occurs in some abundance in the 

 neighbourhood of Thursday Island, the polyps are usually a bright apple-green or primrose-yellow, 

 sometimes varied with a little brown. In another Barrier species, A. spongiosa, the polyps are 



