OYSTEK-CULTUBE IN AUSTRALASIA. 553 



subject to periodical if not permanent reduction by the accession 

 of fresh water. No instance has, in fact, so far fallen within 

 my notice in which this species occurs in permanently pure salt 

 water. There are, however, two species of edible oysters in- 

 digenous to the tropical and sub-tropical Australian regions, 

 which are, on the contrary, exclusively salt-water inhabitants. 

 One of these— Os^rea viordax — having a pinkish triangular 

 crenulated shell, occurs in the greatest luxuriance on the out- 

 lying reefs and islets of Torres Straits and the Great Barrier 

 system, and extends so far south as the Tweed Heads, on the 

 borders of Queensland and New South Wales. The same 

 species is reported to occur throughout Polynesia, and is, I 

 believe, abundant on the sub-tropical coast-line of the North 

 Island of New Zealand. Although excellent eating, this oyster 

 is, unfortunately, of but little commercial value. This draw- 

 back to its utility is occasioned by the fact that the species, 

 adapting itself to its environment, amidst the full force of the 

 breakers, adheres with such tenacity to its rocky support that 

 it cannot be detached without breaking the shell to an extent 

 that precludes its transport in a sound condition to auy dis- 

 tance. From a biogenetic standpoint it is, I think, highly 

 probable that this oyster, Ostrea mordax, represents the type 

 from which the commercial brackish-water form, Ostrea 

 glomerata, was primarily derived. 



The second species of essentially marine edible oyster grows 

 side by side with Ostrea mordax, but is confined more ex- 

 clusively to the tropics. It is usually of an ovate shape, from 

 6in. to 8in. in length, and has a dense white porcellanous shell 

 with a black interior border. It may be easily detached from 

 the rocks with a hammer and cold chisel, and, while somewhat 

 coarse and tough eaten au naturel, is an excellent oyster for 

 culinary purposes. Not having been able to establish the 

 specific identity of this tropical oyster, examples have been' 

 recently remitted by me to the British Museum for identifica- 

 tion."'' 



There are numerous species of oysters indigenous to Australia 

 that are of too rare occurrence to have any commercial value, 

 and are therefore onl}^ of scientific import. Although a detailed 

 description of these non-commercial varieties would be outside 

 the scope of this address, a brief reference may be made to two 

 forms among them, since they represent respectively the 

 largest and the smallest species of true oysters inhabiting Aus- 

 tralian waters, if not, indeed, the universe. The first, or largest, 

 type is allied to the well-known cock's-comb oyster, Ostrea 

 crista-galli, and may be distinguished by the regular zigzag 



* Mr. Brazier, of the Australian ?*Iusemn, Sydney, has recently identi- 

 fied this species for me with the Ostrea nigro-viarrjinata of Sowerby. — S.-K. 



