Dr. G. C. Wallich on the Diatomacea. 7 



tint, — these tufts being intermixed with numerous glistening 

 cyhndrical bodies of a similar colour. A succession of calms, 

 following one on the other, enabled me to trace this phienomcnon, 

 with but slight intermission, from 18° North lat. to 24° South. 



On the Atlantic side of Africa the tufts alone were to be seen, 

 and in smaller quantities, as might be expected from the stormy 

 character of the sea. Without entering into a detailed descrip- 

 tion of these forms, I will merely state that the tufts were of 

 two kinds, — one consisting of cylindrical filaments, closely re- 

 sembling Trichodesmium in character ; the other, of filaments of 

 a Rhizoselenia*. The Hocculent masses, which when seen float- 

 ing on the water resembled flakes of yellow cotton, were from 

 half an inch to two, or even three inches in length, and proved to 

 be aggregations of filaments of the same genus. The cylindrical 

 bodies were gigantic Coscinodisci, each disk being distinguish- 

 able at a considerable distance from the eye, and even for several 

 feet below* the immediate surface. 



I was at first induced to refer the whole of the tufts and floc- 

 culent masses to the Confervoid growths described as frequently 

 occurring in the Red and Yellow Seas in such profusion as to 

 tinge their waters of a reddish-yellow colour. The examination 

 of the Trichodesmium-like tufts, which happened to come first 

 under my notice, confirmed this view for a time ; and it was not 

 until a more extended analysis revealed the presence of the sili- 

 cious forms, that the true character of the latter became appa- 

 rent. Salpce and Diphyes, taken previously in the upper por- 

 tion of the Bay of Bengal, had already brought to light a con- 

 siderable number of Diatomacea;, specimens of v/hich were now 

 to be found entangled in the tufts and flocculi alluded to, im- 

 bedded in the substance of TJialassicolla^ or abundantly distri- 

 buted in the alimentary matter procured from Salpians and 

 other small creatures. 



The mighty scale on which the Diatomaccee really exist did not 

 become manifest, however, until we reached the Atlantic, between 

 the Cape and St. Helena. 



It was here that, for many degrees, and in bright breezy 

 weather, the ship passed through vast layers of sea-water so 

 thronged with the bodies of a species of ^aipa {S. mucronata) as 

 to present the consistence of a jelly. These layers extended for 

 several miles in length. AMiat their vertical limits were, it was 

 impossible to discover, owing to the sj)eed at which the ship was 

 moving. They appeared to extend deep, however, and in all 



* This genus, as originally estaljlished by Elircnberg, conipiiscs forms 

 widely distinct from the genus as now circumscribed by !Mr. Brigbluell of 

 Norwich, and referred to above (Quart. Jouni. Alicroscop. Hoc, vol. vi. 

 p. 93). 



