THALLOPHYTES, BRYINAE. 35 



observed in modern cases. Van Tieghem even believes that he has seen 

 his Bacillus Amylobacter in a silicified state. 



Somewhat better results have been obtained from investigations into 

 fossil Algae. Of these a large number of forms have been described which 

 are either doubtful or of no value to the botanist, and which will be briefly 

 noticed at a later period in this work ; but a certain number of groups may 

 be selected for mention here, since their relations to recent forms can be 

 established with more or less certainty. There are first the Diatomaceae, 

 whose siliceous valves occur in Tertiary and Quaternary deposits in such 

 large masses and so entirely or almost entirely free from admixture, that 

 they form layers several metres thick of a loose white substance known as 

 tripoli-powder, which has been largely employed of late years for technical 

 purposes. The polishing slate of Bilin in Bohemia and of Habichtswald 

 near Cassel, a white stratified rock of Miocene age, is almost entirely com- 

 posed of these Diatom-valves. Each separate deposit of the kind usually 

 contains a large number of species, but these are almost always so disposed 

 that one species or a few form the chief mass of the deposit, and the rest 

 are isolated and disseminated through it. Ehrenberg \ to whom we owe 

 the most searching investigations into fossil Diatoms, states for example 

 that Gallionella distans and Podosphenia nana in alternate layers form 

 almost the entire mass of the polishing slate of Bilin, that Eunotiae and 

 Synedra capitata predominate in the Diatom-earth of Santa Fiora, while 

 the Cassel deposits are to a great extent composed of Naviculae. Ehren- 

 berg 2 has further shown that a large number of Diatoms are found also in 

 the uppermost beds of the Chalk, and gives a list of them 3 ; among them 

 are Fragilaria, Gallionella, Coscinodiscus, Triceratium, Amphitetras, and 

 others. Most of these forms have been found only in the Chalk marls of 

 Oran, Caltanisetta, and Zante, but a few occur in company with Polythala- 

 miae in the true white writing-chalk, for example, Gallionella aurichalcea, 

 Fragilaria rhabdosoma, and Fr. striolata at Rugen and Gravesend. 

 Pfitzer 4 therefore is mistaken when he says that we search in vain for 

 Bacillariae in the Upper Chalk. Ehrenberg 5 has already shown that all 

 these forms down to the Chalk belong to still living genera, that many of 

 them are even identical with recent species, and that the percentage of 

 species not now known to be living diminishes in the strata from below 

 upwards. Even in the Chalk we meet with several species still in existence. 

 The Diatom-beds seem to have been formed both in fresh and salt water; 

 most of the beds of tripoli-powder and Diatom-earth were formed in fresh 

 water, the Chalk-marls in salt water; the latter deposits contain forms 



1 Ehrenberg (1). 2 Ehrenberg (1) and (2). 3 Ehrenberg (2), p. 119. 



4 Pfitzer, die Bacillariaceen in Encyclop. d. Natw., Handb. d. Botanik von A. Schenk, ii, p. 409 (1882). 



5 Ehrenberg (2). 



D 2 



