252 THE GREAT BARRIER REEF. 



of Ostrea glomerata. Adult clusters obtained from a similar depth, moreover, contained both the 

 normal and the elongated types in the same group. 



A third notable variety of the ordinary rock oyster, Ostrea glomerata, is most typical of the 

 northern area of its distribution. It is remarkable for its almost circular, or sub-orbicular, contour, 

 with prominent radiating frillings and, most frequently, a deep cup-like excavation of the attached 

 shell. This type of oyster is abundantly represented in the extensive banks and reefs in the 

 neighbourhood of Keppel Bay, and is also dominant among those types growing on the mangroves 

 in the Endeavour River, near Cooktown. It never attains to the large size of its southern congener, 

 a shell having the capacity of a single fluid ounce being of abnormal dimensions. This modified 

 form would appear to be identical with the type upon which, on insufficient evidence, the so-called 

 Ostrea mytiloides, of Lamarck, has been established. That it is only a variety, however, of the 

 common species, and not a distinct form, is abundantly shown by the fact that oysters approaching 

 the normal triangular outline occur with it ; while the orbicular variation, on the other hand, 

 may not unfrequently be detected growing among the typical triangular southern race. Within 

 the circle covered by the ordinary triangular, the orbicular, and the abnormally elongate 

 modifications of Ostrea glomerata that have now been enumerated, the inclusion of any of the 

 numerous intermediate local variations that occur will be an easy task. 



One somewhat abnormal variety of Ostrea glomerata invites brief notice. This is the form 

 represented by Fig. lo of Chromo plate XIV., conspicuous for the slender spinous processes 

 developed on the surface of its unattached valve, which communicate to it an aspect corre- 

 sponding with that of certain of the Thorny Clams, genus Spondylus. There would appear to be 

 little reason for doubting that this modified form is identical with the so-called Ostrea spinosa 

 of Quoy ; though its correct allocation to the position here allotted it is demonstrated by the cir- 

 cumstance that, where it occurs, every phase of variation between the spined and spineless shells 

 is to be found. It is, moreover, only a young condition, the spines, as the shells grow older, 

 becoming gradually obliterated. In this respect it recalls the earlier growth-phase of the mother- 

 of-pearl shell Meleagrina tnargaritifera, delineated and described in association with the phototype 

 plate No. XXXVIII., wherein projecting lappet-like laminae are profusely developed over the sur- 

 face of the shell. This spinous variety of the young of Ostrea glomerata has been observed by 

 the author most abundantly among colony-growths attached to the aerial roots of the orange 

 mangrove, Rhizophora mucronata, in the neighbourhood of Cairns, and also on rocks at the north 

 end of Wide Bay, and in Keppel Bay. 



OSTREA GLOMERATA.— COLLECTIVE CONDITIONS OF GROWTH. 

 Among the several natural growth-conditions under which the commercial oysters of Queens- 

 land occur in marketable quantity and dimensions, the " Bank " variety, in which the bivalve 

 spreads itself over extensive level banks that are more or less uncovered at low-water, represents 



