OYSTERS AND OYSTER FISHERIES OF QUEENSLAND. 251 



The diagnostic characters of this typical rock oyster, Ostrea glomcrata, as embodied in 

 Reeves's original description, are as follows : — " Shell thick, irregular, sharp-ribbed, with the 

 margin dentated or lobed, very inequivalve ; upper valve opercular, compressed, wrinkled, 

 with thick concentric laminae ; lower valve cucuUated, purple externally, white within, 

 edged with purple or black ; lateral margins denticulated ; hinge generallj' attenuated, pro- 

 duced, pointed." 



That modification of the species which is popularly known as the Dredge or Drift Oyster, in 

 reference to the circumstance that it inhabits deeper water, from which it is collected with the 

 dredge, has had conferred upon it by Sowerl}' the technical title of Ostrea siibtrigona. Its distinc- 

 tive features, as compared with those of O. glomerata, are thus enumerated bj- the authority 

 quoted : — 



"Shell sub-trigonal, oblong, or sub-quadrate, ponderous, rather narrowed toward the umbones, 

 broad at the ventral margin, quadrate ; margin strongl}' plicated, lower valve deep, greenish white, 

 edged slightly with purple ; without, radiately plicated, concentrically banded with fawn and 

 purple ; hinge acuminated, sides crenulated near the hinge. The sculpture of the shell is bold 

 and large, and the square character of the ventral margin is striking." 



Oyster-shells, possessing the characteristic features incorporated in the two foregoing 

 diagnoses, may be found growing side by side in the same cluster, if collected from a bank exposed 

 at ordinary low tide or dredged from a depth of three or four fathoms. A tendenc)' nevertheless 

 prevails, with the shallow water, tide-exposed, racial stock, to develop the more luxuriantly frilled 

 and convoluted marginal border and brighter colours associated with the typical Ostrea glomerata, 

 as illustrated by Chromo plate XIV., Fig. 8; while with those growing in deeper water, a smoother, 

 more ponderous form, with often an abnormally elongated contour, and a more or less complete 

 absence of the conspicuous coloration characteristic of the shells exposed to light and air, is 

 found to predominate. 



In another form, very prevalent among the oysters taken from deep water, or dredge sections, 

 the prolongation and smoothness oi the component shells are more conspicuous)}- pronounced than 

 in the typical dredge or drift variety, associated in the foregoing diagnosis with the title of Ostrea 

 siibtrigona, many specimens collected by the author being no less than three or four times as long 

 as broad. This abnormal elongation, it would seem reasonable to anticipate, demonstrates a 

 disposition on the part of the mollusc to grow upwards towards the light, much after the manner 

 of a light-starved plant. That this tendency to elongate may be manifested at an early period in 

 the oyster's life, was well shown by brood-clusters only a few weeks old, attached vertically to the 

 dead valves of the mollusc Parallelopipedum, which were dredged from a depth of four fathoms in 

 Moreton Bay. The same dredge haul that yielded these specimens brought up, however, a much 

 more considerable number of brood specimens, agreeing strictly in contour with the typical form 



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