OV CEKTAtEOUS PLANTS 235 



fuooids of the Fl3'sch were of physical origin or due to animal 

 tracks. Eothpletz more recently (1896) has taken a moderate 

 view, and maintains the algal nature of many of the species. 

 These papers are not confined to a consideration of the Creta- 

 ceous Algae, but deal with the whole range of forms. 



The Calcareous Algae are naturally, by reason of their hard 

 encrustations, of most importance geologically, and they do 

 really play a part as "rock builders" (see Seward, 1894b). 

 Several undoubted genera and species of this group have been 

 described from Cretaceous rocks. 



Order DIATOMACEaE. 



[Living family.] 



Minute unicellular Algae sometimes living in colonies, but 

 generally free swimming. The shape of the cell is exceedingly 

 various, and particularly characterised by the thickened cell- 

 wall which is in two parts, fitting together like a pill-box and 

 its lid. This shell is generally silioified and often flattened and 

 ornamented. In the living cell the chloroplast has a yellowish- 

 brown colour. 



Possil diatoms are recorded in enormous masses in several 

 Tertiary and a few Cretaceous deposits. In some cases, indeed, 

 a fine whitish earth consists almost entirely of myriads of the 

 minute silicified tests. 



Ehrenberg (1841) pointed out long ago that many species of 

 diatoms existed in the Upper Cretaceous, several apparently 

 identical with living species, and all belonging to still living 

 genera. He gives a list (p. 119) of the species and their localities 

 which is too long, for quotation, but a few names selected from 

 it will indicate the distribution and character of the forms he 

 records : — 



Amphitetras ardediluviana, from the Chalk Marl of Oran, 

 Coscinodiscus Aryus, from the Chalk Marl of Caltanisetta, 

 Coseinodiscus eccentricus, from the Chalk Marl of Oran, 

 Coscinodiscus Patina, from the Chalk Marl of Zante, 

 Frac/ilaria rliahdosoina, from the White Chalk of Gravesend. 



