118 



BacillariesR 



well-known deposit at Richmond, Virginia, U.S.A., is very extensive and 

 attains a thickness of 30 feet, while on some of the geological surveys in the 

 western states of America beds have been discovered no less than 300 feet in 

 thickness and containing 80 per cent, of silica in the form of the frustules 

 of diatoms. Diatoms have been found in the London Clay of the Lower 

 Eocene, and are abundant in the Cretaceous rocks of the Paris basin. 



All the Tertiary and Cretaceous diatoms show a close resemblance to 

 existing species, and in some instances the species are identical. This 

 is particularly the case with the pennate diatoms, but many of the fossil 



2 



Fig. 85. A few fossil diatoms. 2 a d, Pyxidlcula Itollensis Eothpl. and 3 a c, P. liasica 

 Eothpl., both from the Upper Lias. 4 a and b, Stephanopyxis sp. from the Oligocene. 

 4 c, Pyxidicula sp. from the Miocene. 5, Asterolampra Ralfsiana Grev. ; 6, A. marylandica 

 Ehrenb. ; 7, A. crenata Grev. ; 8, A. decorata Grev. All four from the Barbados deposit 

 All x 500. (After 0. Miiller.) 



species of centric diatoms are distinct from the recent ones, and in this 

 section considerably more than twenty exclusively fossil genera are known. 

 Some of the more important are Porodiscus, Actinodiscus, Brunia, Pyrgo- 

 discus, etc. Actinoptychus, although represented by some living marine 

 species, embraces a large number of fossil forms, a fact which is also true 

 of the genus Asterolampra. 



It is probable, as believed by Pantocsek, that the deposit at Kusnetzk 

 in Hungary belongs to the Trias, in which case this is the oldest known 



