80 GENEKAL HISTORY OF THE IXEUSOHIA. 



Akad. 1849), " The chain of rocky mountains traversing the continent of 

 North America, forms, Avith reference to the distribution of Infusoria, a 

 stronger barrier between California and Oregon, and the rest of the continent, 

 than does the Pacific Ocean, with Chraa, between the western plains of 

 North America and the region of Siberia. Thus, the United States, with 

 Mexico, never present any of the forms characteristic of Oregon and Cali- 

 fornia, whilst, on the other hand, the peculiar forms of these latter countries 

 are met mth in Siberia. All this is remarkably confii^med in this, that the 

 gold region of the Sacramento, in the extent and abundance of its Infusorial 

 products, finds its parallel only in Siberia." 



This presumed fact of limited geographical distribution is thus applied 

 by Ehrenberg m another paper (Monatsh. 18-16) : — " The atmospheric dust 

 which, since 1830, has fallen in the Atlantic Ocean as far as 800 miles west 

 from Africa, on the Cape de Yerde Islands, and even in Malta and Genoa, 

 has been all of an ochre-yellow colour, never grey like the dnst seen in the 

 north of Africa, and consists of from -i-th to k'd of organic particles referable 

 to 90 species, the greater number of which are of freshwater habit, and found 

 equally in the most "widely separated regions named. This dust, even in 

 Genoa, whence it is carried by the Sirocco wind, contains no characteristic 

 African forms, but, on the contrary, presents the Si/nedra Entomon, a deci- 

 dedly characteristic species of South America." From his observations on 

 this meteoric dust, Ehrenberg concludes that there is a current of aii' imiting 

 Africa and America in the region of the trade winds, and occasionally dii'ected 

 towards Eui^ope. On the other hand, their wide diffusion is exemplified in Dr. 

 Hooker's Report on the Diatomaceous vegetation of the Antarctic sea {Brit, 

 Assoc. 1847) : — " The genera and species of Diatomaceoe collected within the 

 Antarctic sea are not at all peculiar to those latitudes ; on the contrary, some 

 occur in every country between Spitzbergen and Victoria Land. Others, and 

 even some of these, have been recognized by Ehi-enberg as occurring fossil 

 in both Americas, in the south of Europe and north of Africa, in Tripoli 

 stone and in volcanic ashes ejected both from active and extinct volcanos, 

 whilst others again exist in the atmosphere overhanging the tropical At- 

 lantic." 



Prof. Smith has the foUo^ving remarks on cosmopolitan or very widely- 

 difiused species (Sr/nops. ii. p. xxvii) : — 



" Of freshwater species frequent in the British Islands, the following seem 

 almost cosmopolitan, viz. Si/nedra radians, Pinnularia vir^idis, Pinnidaria 

 horealis, and Cocconema lanceolatwn. Gatherings from many locaHties in 

 Europe, from Smyi-na and Ceylon, from the Sandmch Islands, New Zealand, 

 and New York, from the loftiest accessible points of the Himalaya in Asia, 

 and the Andes in America, have supplied specimens of these forms. 



*' Navicula seriam abound in all our mountain bogs, and is equally common 

 in the marshes of Lapland and America. 



" Epitheinia gihha is an inhabitant of the Geysers of Iceland and the lakes 

 of Switzerland. 



*'The South Sea Islands supply Stauroneis acuta, and Ceylon Synedra 

 Ulna, while Stauroneis Phoenicenteron is equally abundant in Britain, Sicily, 

 and Nova Scotia. 



''These notes of localities will give some idea of the wide distribution of 

 our fluviatile Diatomaceae : more numerous gatherings would, no doubt, 

 greatly extend the list ; and the following circumstance \vi\l show how gene- 

 rally our commoner British forms are diffused throughout European localities 

 that have been carefully examined. During a tour in Languedoc and the 

 Auvergne in the spring of 1854, I made upwards of forty gatherings from 



