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organisms in other parts of the world have had the same effect, 

 indeed there are numerous cases recorded, and probably many- 

 such instances have escaped notice. For example, had the 

 organism which appeared in Port Jackson been colourless its 

 presence might have been overlooked, and the destructive effects 

 produced by it would have remained enshrouded in mystery. As 

 a proof of this I may mention the fact, that another and allied 

 species {Gymnodinium spirale, Bergh,) made its appearance during 

 the period in which the Glenodinium reached its climax, and in 

 a short time it appeared to equal the latter in number in many 

 parts of the harbour. Yet it remained unnoticed by the public 

 generally, owing no doubt to its being colourless, and had it not 

 been accidentally met with during my examination of the water 

 its advent would probably have been unrecorded. 



From my observations of the habits of the Gymnodinium, I 

 am of the opinion that it had a very important bearing on the 

 final disappearance of the organism causing the discolouration, 

 inasmuch as nearly every individual had in the gastric cavity one 

 or two specimens of the Glenodiniujn, while many of them were 

 so gorged with food as to be almost unrecognizable, being forced 

 wholly out of shape by the contents of the stomach. The 

 Gymnodinium spirale, Bergh, was first observed about the middle 

 of April in water from Tarban Creek, only a few specimens being 

 seen ; during the latter part of the month they seemed to increase 

 rapidly, and by the second week in May they were found in great 

 abundance in many parts of the harbour, the water appearing to 

 be quite thick with them, and it was only necessary to dip a 

 tube into the water to secure some thousands of specimens. 

 Concurrently with the increase of the Gymnndi7mi7n, the 

 Glenodinium gradually disappeared. The above mentioned facts 

 point to the conclusion that the former organism was mainly 

 instrumental in considerably reducing the number of the latter, 

 and no doubt contributed largely towards its final disappearance. 



The sudden discolouration of the harbour by the appearance of 

 a minute organism in such vast numbers is rather a difficult 

 matter to explain, except on the supposition that the whole of 

 the conditions favourable to its full development were nearly 

 perfect, and the causes which would tend to retard it were 

 reduced to a minimum. There is no reason to suppose that this 

 was the first time that the organism had appeared in Port 

 Jackson. Mr. J. Brazier informs me that he has noticed similar 

 discolourations at various times, and in the year 1866 the oysters, 

 mussels, itc, were killed in large numbers. No doubt the causes 

 which contributed towards the development of the organism 

 were exceptionally favourable, the very large rainfall and the 

 remarkable period of calm weather that prevailed during the 

 whole of the visitation, may have had some bearing on its 

 appearing in greater numbers than in other seasons. If the 



