300 OCCURRENCE OF DIATOMS, RADIOLARIA AND INFUSORIA, 



Subsequent examination convinced us that many of these forms 

 were Diatoms, and some probably Infusoria. 



{a) Diatoms. — Diatoms in rocks older than the Tertiary being 

 of comparatively rare occurrence, the literature on the subject is 

 not extensive, so that a short reference to the principal papers 

 may be given here. 



Summarising what is at present known about the fossil 

 Diatomacese, Seward"^ says : — "With the exception of two species 

 of Liassic Diatoms, no trustworthy examples of the Diatomaceca 

 have been found below the Cretaceous Series. The oldest known 

 Diatoms were discovered by Rothpletzf among the fibres of an 

 Upper Lias Sponge from Boll in Wiirttemberg. . . . Roth- 

 pletz describes two species which he includes in the genus Pyxi- 

 dicula, P. bollensis and P. liassica.'^ 



The siliceous frustules referred to these species occur in great 

 numbers, associated with coccoliths, among the horny fibres of 

 the fossil sponge Phymatoder ma. The frustules are thimble- 

 shaped, minutely punctate or perforate {apparentl}'- the latter to 

 judge from the figures), and measure in greatest diameter 6-14 ^. 

 They are usually isolated, but very rarely are met with in pairs 

 united at their open ends, the two portions in this case being of 

 unequal size. They do not, however, overlap one another, neither 

 is any girdle present. 



Rothpletz compares these forms with the genus Stejjhanojjyxis 

 (Schiitt) and the sub-genus Pyxidicula (Schiitt). The former 

 possesses spines on both valves, while the latter is devoid of 

 spines. Rothpletz figures (op. cit., p. 911) a form of Stephano- 

 pyxis from the Oligocene marl of Thisted in Denmark. Many 

 Cretaceous Diatoms have been figured by Ehrenberg| from the 



* Seward, Fossil Plants, Vol. i. , p. 154. 



t Rothpletz, Ueber die Flysch-Fucoiden und einige andere fossilen Algen 

 sowie iiber liasische Diatomeen fiihrende Hornschwamme. Zeits. Deutsch. 

 Geol. GeselL, 1896, xlv., pp. 854-914. Also, Nachtrag zu meinem Aufsatz 

 iiber einen neuen Jurassischen Hornschwamme und die darin eingeschloss- 

 enen Diatomeen. Op. cit., 1900, lii., pp. 388-389. 



ij: Ehrenberg (C. G.), Mikrogeologie. Leipzig, 1854. Und Forsetzung. 

 Idem, 1856. 



