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



become distributed through the water, and are lield in suspen- 

 sion by it, subsidini^ only after some httle time has elapsed/^ 



In the article on JDiatomaccEe in the ' Micrographic Dictionary/ 

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

 species " from the bottom, or from pieces of wood-work, &c., 

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

 entangled in the meshes of Confervse and other Algae, 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 Diatoraacese 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 Diatomaceffi 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 ' 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 i;he embryonic shells of the pelagic Mol- 

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

 Poraminifcra, and a great variety of Diatomacea;, and even Dcs- 

 midica;*, including the genus Closterium, and all apparently 

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



* In the plate accompanvinp; Mr. Macdonakl's paper, a Closterium-\\ke 

 body is ropi'cseutcd, and referred by the author to the family named. A 

 simihir form has rejieatedly been ol)tained by me from the same source. 

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

 blance to a (losterium, but I was unable to detect cither the terminal 

 vesicles or the central suture. 



