246 THE GREAT BARRIER REEF. 



While enjoying so extensive an area of distribution, Ostrea mordnx may be said to attain to its 

 finest or maximum development among tho coral-reefs and islets of the tropical coast-line of eastern 

 Australia ; and from its remarkable abundance in this region it may be appropriately, as it is here- 

 after, distinguished by the suggestive popular title of the Coral-Rock Oyster. In contradistinction 

 to the ordinary rock oyster of commerce, this coral-rock species is an essentially marine type, 

 attaining to its most luxuriant growth among the reefs and islets of the Great Barrier system, far 

 remote from fresh-water influences, and rarely overstepping the boundary limit of pure salt-water 

 in its area of distribution along the rocky headlands on the mainland seaboard. In this latter 

 situation it not unfrequently grows as abundantly as, and under conditions closely simulating those 

 associated with, the rock variety of Ostrea glomerata. The zone of growth most affected by Ostrea 

 tnordax on either the natural rocks, conglomerate reefs, or stranded coral boulders, is coincident 

 with that of half-tide mark ; and the author has not, so far, either by practical investigation or 

 inquiry, obtained information concerning any instance in which this 03'ster has been found 

 growing beneath, or even at as low a level as, ordinary low-tide mark. 



The characteristic features of Ostrea mordax, in its most typical form, are its normally elongate- 

 triangular contour, the very evenly lobate edges of the interlocking shells, and the opaque purplish- 

 pink hue of their external surface. Another peculiarity of this species, as compared with the 

 ordinary commercial oyster, O. glomerata, which it often closely resembles in size and shape, is the 

 circumstance that it is almost always affixed to its rocky support by its left valve, while the last- 

 named form is as invariably affixed by its right one. The flattened, freely movable, so-called oper- 

 cular valve, is, in a corresponding manner, developed on the right side in Ostrea mordax, and on the 

 left one in O. glomerata. An additional point of distinction between the shells of these two species 

 is furnished by the circumstance that the scar or impression of attachment of the large adductor 

 muscle is in O. mordax set very much farther back, or towards the distal or growing edges of the 

 shells, than in O. glomerata; this scar, moreover, in the upper or "opercular" valve is more 

 usually of a deep-black hue, while in the last-named species, though occasionally of a darker tint 

 than the surrounding nacre, it is more often colourless. 



While the shape above described represents the most typical form of Ostrea mordax, it is 

 subject to almost as considerable a latitude of variation as the ordinary commercial oyster, 0. 

 glomerata. This is due to the creature's efforts to adapt its shape to that of its environment, 

 and the shells in consequence vary, from an almost circular to an abnormally elongate contour. 

 A yet more conspicuous modification of this very variable species is one in which the hinge or butt- 

 end of the attached valve is produced into a long hollow beak, which is found, on opening the 

 oyster, to contain a very considerable amount of " meat." This beaked variety would appear to 

 be identical with the form upon which the specific title of Ostrea cucullata was originally con- 

 ferred ; and illustrations of both it and the normal form of Ostrea mordax, are given in Figs, i 

 and 2 of Chromo plate No. XIV. One characteristic peculiarity of the species under notice, 



