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



become distributed through the water, and are 'held in suspen- 

 sion by it, subsiding only after some little time has elapsed." 



In the article on Diatomacese in the ' Micrographic Dictionary/ 

 after a description of the methods for obtaining the fresh-water 

 species " from the bottom, or from pieces of wood-worl^ &c.> 

 immersed in the water," it is stated that " many of them arc 

 entangled in the meshes of Conferva? and other Algse, or on the 

 submerged stems of higher plants, -the deep-sea species" being 

 " obtained by dredging or by treating the alimentary canal of 

 fishes, Mollusca, &c., with acid." 



With two exceptions, immediately to be noticed, the above 

 extracts embrace, as far as I am aware, a summary of the views 

 entertained by writers on the subject ; and they clearly indicate 

 that none of the Diatomacese have, heretofore, been recognized as 

 strictly free-floating organisms, but, on the contrary, that such 

 forms as occur at times suspended in the water are considered as 

 having been removed accidentally from their natural positions, and 

 therefore evincing an invariable tendency to subside to the bottom. 



Dr. J. D. Hooker was the first to notice, the vast profusion 

 of Diatomacese in the South Polar Ocean ; and he pointed out 

 their conspicuous appearance when imbedded in the substance 

 of the ice or washed up on its surface by the action of the 

 waves. 



Still more recently, Assistant- Surgeon Macdonald, of H.M, 

 Ship ' Herald/ in a brief but interesting paper on "Deep 

 Soundings in the South Pacific " (published in the f Annals of 

 Nat. Hist/ for October 1857) offered the subjoined remarks on 

 the subject : - 



" Having ascertained with a certain degree of precision the 

 nature of the material to be found in deep soundings off the 

 coast of Australia and in the neighbourhood of the South Sea 

 Islands, it is a discovery of peculiar interest to find the same 

 minute organic forms, in vast numbers, mixed with the alimen- 

 tary matter of Salpians and other pelagic animals observed in 

 the open ocean, far distant from their shores. 



" The presence of the silicious spicula and the fenestrated cells 

 of Thalassicolla with the embryonic shells of the pelagic Mol- 

 lusca might be readily accounted for. But how minute bivalves, 

 Eoraminifera, and a great variety of Diatomacese, and even Des- 

 midiese*, including the genus Closterium, and all apparently 

 recent, could have been, as it were, casually inhaled, is not so 



* In the plate accompanying Mr. Macdonald's paper, a Closterium-\\ke 

 body is represented, and referred by the author to the family named. A 

 similar form has repeatedly been obtained by me from the same source. 

 In external characters and colour it certainly exhibits the closest resem- 

 blance to a Closterinm, but I was unable to detect either the terminal 

 vesicles or the central suture. 



