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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 spirals , 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 Glenodinium , 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 spirals , 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 Gymnodinium , 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, ifec., 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 
