. Algæ. Vol. I. Myxophyceæ, Peridinieæ, Bacillarieæ, Chlorophyceæ, together with a brief summary of the occurrence and distribution of freshwat4er Algæ . and. The famous deposit at Biln, inBohemia, which averages 14 feet in thickness, was estimated by Ehrenbergto contain some 40,000,000 frustules of diatoms in every cubic inch. Itattains in places a thickness of 50 feet and is hard and flinty, the cementingsilica having been formed by the solution of some of the diatom valves. The 118 BacillariesR well-known deposit at Richmond, Virginia, , is very extensive andattains a thickness of 30
. Algæ. Vol. I. Myxophyceæ, Peridinieæ, Bacillarieæ, Chlorophyceæ, together with a brief summary of the occurrence and distribution of freshwat4er Algæ . and. The famous deposit at Biln, inBohemia, which averages 14 feet in thickness, was estimated by Ehrenbergto contain some 40,000,000 frustules of diatoms in every cubic inch. Itattains in places a thickness of 50 feet and is hard and flinty, the cementingsilica having been formed by the solution of some of the diatom valves. The 118 BacillariesR well-known deposit at Richmond, Virginia, , is very extensive andattains a thickness of 30 feet, while on some of the geological surveys in thewestern states of America beds have been discovered no less than 300 feet inthickness and containing 80 per cent, of silica in the form of the frustulesof diatoms. Diatoms have been found in the London Clay of the LowerEocene, and are abundant in the Cretaceous rocks of the Paris basin. All the Tertiary and Cretaceous diatoms show a close resemblance toexisting species, and in some instances the species are identical. Thisis 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. liasicaEothpl., both from the Upper Lias. 4 a and b, Stephanopyxis sp. from the c, Pyxidicula sp. from the Miocene. 5, Asterolampra Ralfsiana Grev. ; 6, A. marylandicaEhrenb. ; 7, A. crenata Grev. ; 8, A. decorata Grev. All four from the Barbados depositAll x 500. (After 0. Miiller.) species of centric diatoms are distinct from the recent ones, and in thissection considerably more than twenty exclusively fossil genera are of the more important are Porodiscus, Actinodiscus, Brunia, Pyrgo-discus, etc. Actinoptychus, although represented by some living marinespecies, embraces a large number of fossil forms, a fact which is also trueof the genus Asterolampra. It is probable, as believed by Pantocsek, that the deposit at Kusn
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