CARTER, ON THE COLOURING MATTER OF THE RED SEA. 183 
shore-sea of the south-east coast of Arabia from Muscat to 
Aden, where, under its survey, I passed all the months of 
the years 1844-45, and of 1845-46, with the exception of 
those of the stormy monsoon, viz., June, July, August, and 
September, the presence of the scum above described never, 
to my knowledge, was once observed. J am, therefore, in- 
clined to infer that it is chiefly confined to the sea some dis- 
tance off shore. Yet Ehrenberg, in 1823, saw the Bay of 
Tor covered with it, even up to the sands. 
Lastly, 1 would advert (but, as before stated, with much 
diffidence) to that part of the generic characters of Tricho- 
desmium Ehrenbergit in which we find the expression “ prime 
rubro-sanguinee, tandem virides,” first used by Montagne 
(J. c., p. 8346), and then repeated by Kiitzing in his ‘ Species 
Algarum,’ because the facts connected with the accounts 
given of those who have seen the scum formed by Tricho- 
desmium, together with my own experience of Algz generally, 
lead me to the opposite conclusion, viz., that Trichodesmiwn 
is at first green, and subsequently becomes red. 
It is true that its chief colour in the Bay of Tor, when 
seen by Ehrenberg, was red; it was red, like ‘red sawdust,” 
when seen by M. E. Dupont in the Red Sea (ap. Montagne, 
l. c.); but, on the other hand, what I saw in the Gulf of 
Aden and in the Red Sea, together with what Mr. Latimer 
Clark saw in the Sea of Oman, and Dr. Haines, as above 
stated, in the Indian Ocean, was nearly all of a yellow oily 
colour; and this is the appearance that I have heard gene- 
rally assigned to it by those who have been in the habit of 
traversing the seas mentioned. 
Next to the yellow colour, red is the most prevalent, and 
green least of all. Some of that seen by Ehrenberg was 
intensely green ; this was the case also with the green portion 
that I saw with the red above noticed ; while Ehrenberg saw 
other portions of a less green colour. So much for what has 
been stated respecting the colours under which Trichodesmium 
has appeared. 
We come now to the usual course presented by other Algze 
in arriving at a red colour. If we take the Peridinium which 
colours the sea red on the shores of the island of Bombay, we 
shall find, as above stated, that it is at first green, then yel- 
lowish, and lastly red. In the green stage, the contents of 
the cell are so thin and watery that they easily allow the 
hight to traverse them, and thus the Peridinium passes un- 
observed; but as they become inspissated, oil-globules gene- 
rated, and the chlorophyll changed first to yellow and lastly 
to red, these contents become more opaque; and thus the 
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