Mr. J. Ralfs on the Diatomaceae. 449 



The Cymbellea have a siliceous covering, their form therefore 

 is not altered in drying, nor are they destroyed by fire. When 

 in perfection they are generally brownish, and not unfrequently 

 become greenish when dry. They are usually either of a qua- 

 drilateral or a prismatic form, and often marked with striae and 

 puncta. 



The striae are useful in forming the generic and specific cha- 

 racters, and are best examined by subjecting a portion of the spe- 

 cimen to a red heat, which, by destroying the internal colouring 

 matter in all, as well as the mucous connecting portion in Diatoma 

 and its allies, and the covering in Schizonema, renders the figure 

 and markings of the frustule more evident. 



The method of examination here described has been advan- 

 tageously employed by Ehrenberg and other naturalists in the 

 comparison of fossil with recent species, since it deprives the latter 

 of all but the siliceous covering ; but I am not aware that it has 

 been used by any British algologists in the investigation of recent 

 specimens*. 



As the presence of silica is an invariable characteristic of the 

 Cymbellea, its absence in Hydrurus justifies Mr. Harvey's removal 

 of it from the Diatomacea, but we must on the same account ex- 

 clude some plants placed among them in his ' Manual of British 

 Algae.' 



All the genera placed by Mr. Harvey in his ' Manual' under Dia- 

 tomacece will be included in Cymbellea, with the exception only 

 of Desmidium. 



Diatoma, Ag. (Bacillaria, Ehrenb.) 



Filaments flat or compressed, free ; frustules quadrangular, par- 

 tially separating and cohering by the angles, generally by the 

 alternate ones. 



This genus is distinguished from Exilaria, Striatella, Achnan- 

 thes and Isthmia by its unattached filaments, from Biddulphia by 

 the angles not being produced, and from Fragilaria by the con- 

 nexion of the frustules at their angles in a zigzag chain. 



The second section (Tabellaria) appears to be distinguished from 

 Ehrenberg' s genus Tessella by a transverse canal interrupting the 

 striae. 



The corrections in the nomenclature are made on the authority 

 of Mr. Shuttleworth. 



In this as well as other genera of the Diatomacea sufficient at- 

 tention has not been paid by British botanists to the lateral view, 

 which not only assists in ascertaining the shape of the frustule, 

 but often affords good specific distinctions. 



* In this, as well as in other instances, my residence at a distance from the 

 sources of information must excuse my omission of references. 



