12 THE GREAT BARRIER REEF. 



dry with every spring-tide, and may be appropriately compared with an ordinary boulder-strewn 

 beach on the British coast. Rock boulders are, however, here replaced by coral-stocks, while 

 leathery, bright-tinted Alcyonaria growing upon them, take the place of the more familiar olive- 

 brown seaweeds, or Fuci, of the British seas. The coral-masses thickly scattered throughout 

 this reef-view belong almost exclusively to a single species of Goniastraea, very nearly allied 

 to the form illustrated in the two plates previously described. Sprinkled here and there among 

 the Goniastraea, the somewhat larger-celled coralla of another Astr^id, or star-coral, Prionastraea 

 sp., may be detected. Two initial growths, or it may be the isolated, last surviving, fragments, 

 of an original massive corallum of this variety are conspicuous near the summit of the much- 

 eroded block towards the left in the immediate foreground. In life, as shown in Fig. 8 of 

 Chromo plate No. V., the centre of each polyp-cell, or corallite, is brilliant grass-green, the 

 surrounding septal ridges being dark brown. 



The eroded block that forms the basis of these several coralla possesses an intrinsic feature 

 of interest. Securely embedded within its substance, some half-a-dozen specimens of the so-called 

 Frilled, or Furbelow, Clam-shell, Tridacua comprcssa, may be observed. The shells are usually 

 pale yellow, while the membrane, or " mantle," of the living animal, exposed to view betwixt these 

 slightly gaping " valves," is resplendent with a variety of the most gorgeous hues. Some idea of 

 the life aspect of these Frilled Clams may be gained by a reference to No. XIII. of the coloured 

 plates, in which the entire lower portion of the picture is devoted to a representation of some 

 of the more characteristic tints and markings with which these Molluscs may be arrayed. Every 

 gradation of shade, from palest turquoise to the richest ultramarine, and peacock-blue or green, 

 variegated with black spots and scribblings, commonlj' obtain. In other instances the ground 

 colour is purple or rich brown, brilliant green or blue spots and streaks taking the place of the 

 black markings of the series first referred to. This frilled clam, in comparison with its huge 

 relation, Tridacua gigas, found growing on the same reefs, but usually farther out to sea, is of 

 relatively small dimensions, rarely exceeding a length of ten or twelve inches, those embedded in 

 the worn coral-block illustrated measuring about six. The manner in which these clams become 

 so deeply embedded in the coral-masses has been a matter of conjecture. Although the process 

 has not actually been observed, it may be taken for granted that the clams anchor themselves in 

 some small crevice of the corallum selected, at a very minute and early stage of their existence. 

 The pressure of their hard valves acts as a check upon the ingrowth over them of the soft-fleshed, 

 coral-secreting polyps ; while, at the same time, it seems highly probable that, as in the case of 

 many other shell-fish, they possess the property of eroding, or possibly of chemically dissolving, 

 the dead coral matrix m their rear. On examining the picture with an ordinary hand-glass, 

 one detects specimens of these clams established on man}' other coralla ; and it is a moot 

 point whether these shells do not contribute extensively towards the death and dismemberment 

 of the corals upon which they have literally obtained a foot-hold. The encrusting Alcyonaria, 



