P. T. Cleve: The Diatoms of Finland. 13 



Sa. — Savolaks. 



Sat. — Satakunta. 



Sb. — Northern Savolaks. 



Sinb. — Bay of Bothnia. 



Sinf. — Bay of Finland. 



Ta. — Tavastland. 



Tb. — Northern Tavastland. 



Systematic arrangement of the diatoms. 



In order that the classification of the Diatomaceas may be 

 something more than a mere list of names, it is necessary to dis- 

 pose the forms according to their natural affinities, so that the 

 arrangement may represent as far as possible the succession of 

 the evolution of the forms. 



It is known from palaeontological researches that primordial 

 forms are as a rule complex in their characteristics, and that they 

 bear a more or less close affinity to different groups. In the 

 Diatomacese it appears that the most complex characteristics are 

 to be found in the family of the Nitzschieaa. This family may 

 therefore be regarded as the centre of the system, and from this 

 radiate all the other groups of diatoms. 



The genus Hantzschia and the section Pseudoepithemia sug- 

 gest a passage to Epithemia. On the other hand Cymatopleura 

 seems to be akin to Tryblionella. Stenopterobia and Surirella 

 connect the Nitzschiese with the genus Gampylodiscus. The di- 

 vision Grunowia is closely related to Denticula. Thalassio- 

 thrix seems to connect the Nitzschiese with Svnedra and Fragi- 

 laria, from which gradual passages may be traced to Tabellaria 

 (Grammatophora, Rhabdonema), Eunotia, Diatoma (Terpsinoe 

 Biddulphia, Hemiaulus, Cha3toceras, Rhizosolenia). 



It is generally admitted that the passage from the Nitzschieee 

 to the Raphidese is formed by Amphiprora and its allied genera, 

 especially Plagiotropis. The resemblance between Perrya, Pseu- 

 doepithemia and Amphiprora is remarkable. The passage between 



