154 THALLOPHYTA. [CH. 



many cases the species are identical, and the fossil deposits as a 

 whole seem to differ in no special respect from those now 

 being built up. 



With the exception of two species of Liassic Diatoms, no 

 trustworthy examples of the Diatomaceae have been found 

 below the Cretaceous series. The oldest known Diatoms were 

 discovered by Rothpletz^ among the fibres of an Upper Lias 

 sponge from Boll in Wtirtemberg. They occur as small 

 thimble-shaped siliceous tests with coccoliths and foraminifera 

 in the horny skeleton of Phymatoderma, a genus formerly 

 regarded as an alga. Rothpletz describes two species which 

 he includes in the genus Pyxidicula, P. hollensis and P. liasica. 

 This generic name of Ehrenberg is used by Schtitt^ as a sub- 

 genus of Stephanopyxis. 



Seeing how great a resemblance there is between the recent 

 and Cretaceous species, and how many examples there are of 

 Tertiary diatom deposits, it is not a little surprising that the past 

 history of these plants has not been traced to earlier periods. 

 In 1876 Castracane^ an Italian diatomist, gave an account of 

 certain species of diatoms said to have been found in a block of 

 coal from Liverpool obtained from the English Coal-Measures. 

 The species were found to be identical with recent forms. It 

 is generally agreed that these specimens cannot have been 

 from the coal itself, but that they must have been living forms 

 which had come to be associated with the coal. The late Prof. 

 Williamson spent many years examining thin sections and 

 other preparations of coal from various parts of the world, but 

 he never found a trace of any fossil diatom. There is no 

 apparent reason why diatoms should not be found in Pre- 

 Cretaceous rocks, and the microscopic investigation of old 

 sediments may well lead to their discovery. Prof. Bertrand 

 of Lille, who has devoted himself for some time past to a detailed 

 microscopical examination of coal, informs me that he has so 

 far failed to discover any trace of Palaeozoic diatomaceous tests. 



The genus Bactryllium is often quoted in text-books as a 

 probable example of a Triassic diatom. It was first described 



1 Kothpletz (96) p. 910, fig. 3, PL xxiii. fig. 203. 



2 Schtitt (96) p. 62. 3 Castracane (76). 



