24 CEITISH DESMIDIAOE^!. 



The arrangement of the genera of Desmids given in 

 this work is based upon the scheme of evolution 

 already published by one of the authors, and upon 

 certain recent investigations of Liitkemiiller. 



The division of Desmids into solitary forms and 

 filamentous forms can no longer be followed. The 

 genera Gonatozygon and Cfenicularia, although more 

 specialised than Spirotaenia, Mesotsenium, or Netriinii, 

 have so little in common with most other Desmids that 

 they must be regarded as having had a distinct origin 

 from filamentous ancestors. Most of the genera of 

 Desmids appear to have evolved from Oylindrocystis 

 and Mesotsenium., which were themselves derived from 

 filamentous ancestors, through such Conjugates as 

 Debarya desmidioides. The genus Peniinn undoubtedly 

 contains a collection of widely different plants, some 

 of which should be distributed in the Spirotsenieas, 

 Closterieas, and Cosmariese ; and Liitkeiniiller's 



tion that Nageli's genus Netrium be used to include 

 certain well-marked forms is a most useful one. The 

 true Penia are of the nature of Penium margaritaceuru, 

 P. Cylindrus, P. spirostriolatum, etc. 



The genera Docidium and Pleurotsenium belong- 

 unquestionably to the tribe Cosinariese, but their 

 origin is doubtful. 



The retention of the names Pleurotseniopsis and 

 Pleurenterium as genera of Desmids is as useless as it 

 is foolish. These were originally proposed as sub- 

 genera by Lundell and afterwards elevated to the rank 

 of genera by other less experienced authors. The 

 establishment of such genera is against all principles 

 of natural classification, as it necessitates placing in 

 one genus a few of the most diverse forms of the 

 present genera Gosmarium and Staurastrum, forms 

 which have obviously no direct relationship with each 

 other. Similarly, the genus Dyxph iiirfium Nag. ( Calo- 

 cylindrus Kirchn.) has no definite line of demarcation 

 from Gosmarium Corda. 



The large genera Gosmarium, and Stawastrum may 



