Dr. Collingwood, on Microscopic Alga. 91 



mens, also, he relates, sank to the bottom of the glass during 

 the night, rising again in the heat of the day. I never ob- 

 served any phenomenon approaching to this. They always 

 floated in the water for the most part, but some few seemed 

 to have greater specific gravity, and sunk to the bottom. In 

 the ocean, I have observed the scum on the surface in early 

 morning and at sunset; but in the cases of the sparkling 

 appearance in the sea, the fasciculi hovered at various depths 

 below the surface, although it was during the heat of a 

 tropical day. 



Montague appends to his exhaustive paper in the ' Annales 

 des Sciences' a series of conclusions on what was known, 

 and questions for further observation, most of which are 

 referred to, and answered in, the present pajier ; but there 

 still remains the curious fact that although three species are 

 described, T. erythreeum, T. Ekrenbergii, and T. Hindsii, 

 they are all three spoken of as blood-red — a colour which I 

 have never seen approached. Again, one of the generic 

 characters of Trichodesmium given both by Ehrenberg and 

 Montagne is " muco involuti," while I confidently state that 

 no mucous envelope characterised the species so abundant in 

 the China Sea, and which I also observed in the Indian 

 and Atlantic Oceans. But, then, it might be said the ex- 

 planation is easy, viz., that the China Sea Alga is of a 

 difierent sj^ecies from that of the Red Sea. I have no doubt 

 whatever that this is the case, but the Alga met with by 

 Darwin near the Abrolhos islets, which gave the sea "a. 

 reddish-brown appearance," and which, from his description 

 of it, was apparently the same as that I so abundantly met 

 with in the China Seas, was pronounced by Mr. Berkeley to 

 be Trichodesmium erythr(Eum, " the same species with that 

 found over large spaces in the Red Sea." It is true Mr. 

 Darwin describes it as a reddish-brown, but he elsewhere 

 states that the endochrome was of a brownish-green — which 

 is more suggestive of the colour, as I have always seen it. 

 So also the substance seen by Banks and Solander in the 

 neighbourhood of New Guinea was doubtless what I have 

 described, and the name universally given to it by Cook's 

 sailors, viz., sea sawdust, exactly expresses its appearance 

 and colour, implying, however, nothing red. 



With the exception, indeed, of the observations of Dr. 

 Hinds, the blood-red Alga seems nowhere to have been met 

 with but in the Red Sea and Arabian Gulf, and it would, 

 indeed, be strange if the same Alga Avas always blood-red in 

 the Red Sea, and yellowish-brown somewhere else. More- 



VOL. XVI. /* 



