4, 



view of the conjoined shells, showing their characteristic curvature, are given at Plate IV., Figs. 4 and 5. 

 It not having been found possible to identify this oyster vnth any of the species hitherto included in the 

 Australian list, it is here proposed to provisionally associate with it the title of the Saddle Oyster — 

 Ostrea sellaformis. The largest specimens so far observed measured as much as 4 or 5 inches in 

 diameter, or a size calculated to yield a substantial amount of meat. Although met with as isolated 

 individuals only in Moreton Bay, it may not improbably grow in some profusion and in an associated 

 condition in the outside waters. 



Among the species of oysters occurring locally on the Queensland coast, but which are of too small 

 a size for commercial purposes, T may make mention of the Ostrea spinosa of Quoy, which I have 

 observed in some abundance on the rocky shores of Curtis Island in Keppel Bay, and also in Wide Bay. 

 It adheres firmly to the rocks, and is covered with elongated spinous processes, somewhat resembling 

 those of the Thorny Clams, genus Spondylus. A small, corrugated, red-shelled oyster that grows sparingly 

 in deep water in various parts of Moreton Bay is known technically by the title of Ostrea circumsuta. 



A species of bivalve classed among oysters in popular terminilogy, but which belongs to the 

 genus Spondyhis, previously referred to, demands brief attention with reference to the fact that it is 

 very unwholesome eating, if not absolutely poisonous. The species, while occurring as far south as 

 Moreton Bay, is most abundant among the coral reefs of the tropical coast-line. It may be easily recognised 

 by the symmetrical ovate shape and convexity of its component shells, the lower or attached valve of the 

 two being particularly deep and cup-like, and by the peculiar formation of the hinge-joint, the shelly 

 teeth of which so interlock with one another that, while the valves open and close with the greatest 

 readiness, they cannot be separated from one another without force, even after the death of the animal 

 and disintegration of the connecting ligaments. The flesh of this false oyster when opened is of a pale 

 pinkish hue. If partaken of in any quantity, in mistake for some ordinary edible varietv. severe purging 

 and nausea usually ensues. In order to further facilitate the recognition and avoidance of this unwhole- 

 some species, a characteristic figure of it is given at Plate II., Fig. S. 



THE ORDINARY COMMERCIAL OYSTER, OSTREA GLOMERATA~lT^ INDIVIDUAL 



MODIFICATIONS. 



The ordinary commercial oyster of Queensland, with which the appellation of the " Eock Oyster" is 

 most popularly associated, is best known scientifically as the Ostrea glomerata, of Gould. This bivalve, 

 however, is subject to such an infinity of individual variations, dependent upon its special conditions of 

 growth and environment, that some concologists have been led to confer a separate specific name upon 

 each most prominent variety. Since, however, intermediate modifications abound that unite these at 

 first sight divergent types into one harmonious series, the latest scientific opinion is in favour of 

 including the whole within the single specific title above cited. 



The diagnostic characters of this typical rock oyster, Ostrea glomerata, as embodied in Eeeves'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 lamince ; 

 lower valve cucullated, purple externally, white within, edged witli purple or black; lateral margins 

 denticulated ; hinge generally attenuated, pro.duced, pointed." 



That modification of the species that is popularly known as the dredge or drift oyster, with 

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

 has had conferred upon it by Sowerly the technical title of Ostrea siibtrigona. Its distinctive features 

 as compared with those of O. (jJomerata are thus enumerated by the authority quoted : — 



"Shell subtrigonal, oblong, or subquadrate, ponderous, rather narrowed towards the umbones, broad 

 at the ventral margin, quadrate ; margin strongly 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, collected from a bank exposed at ordinary low 

 tide, or if dredged from a depth of three or four fathoms. A tendency, 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, while with those growing in 

 deeper water a smoother and more j)onderous form, with often an abnormally elongated contour, and 

 a more or less complete absence of the conspicuous colouration characteristic of the sheDs exposed to 

 light and air, is found to predominate. The several specimens figured in Plate 1 have been selected for 

 the purpose of illustrating some of the most characteristic modifications of this exceedingly variable 

 species. At Fig. 1 is represented a cluster of the most esteemed marketable form, or typical 

 glomerata, taken off a cultivated bank ; Figs. 2 and 3 depict the concave adherent or so-called lower 

 valve, and the flat, movable, or opercular valve of a similar oyster separated in order to show the internal 

 structure and position of the muscular scar. 



Fig. 4 has been chosen to illustrate a form very prevalent among the oysters taken from deep 

 water or dredge sections, and in which the prolongation and smoothness of the component shells are more 

 conspicuously pronounced than in the typical dredge or drift variety associated in the foregoing diagnosis 

 with the title of Ostrea sublrigona. This abnormal elongation, it would seem reasonable to anticipate, 

 exhibits 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 is well showu by the brood-cluster represented at Fig. 5 of the same Plate, and in 

 which a number of slender elongated shells are attached vertically to the dead valve of a Parallelopipidon. 

 These young oysters were dredged from a depth oi four fathoms in Moreton Bay. The same dredge haul 

 that yielded these specimens brought up, however, a much more considerable number of brood agreeing 

 strictly in contour with the typical form of Ostrea glomerata. Adult clusters obtained from a similar 

 depth, moreover, containing the normal and the elongated modification in the same group. 



A third notable varietv of the onlmary rock oyster, Ostrea glomerata, is most tyjjical of the northern 

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

 radiating friilings and, most frequently, dcc'p 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 



