Desmidiece.'] desmidie^. 361 



1. Zygnema auiNiNUM, Agardh. 



The filaments are pale yellowish-green ; the spires perform 

 three revolutions in each cell ; the spores are simple. Yery 

 common in ditches and pools^ in cloudy masses, of a pale 

 green colour ; filaments glossy, and marked with a spiral 

 line resembling a constant repetition of the Eoman numeral 

 Y., or five, w^hence the specific name quinhmm. This is 

 the chief distinguishing mark from another very common 

 species, which has two spiral lines crossing each other, and 

 thus repeating throughout its whole length the Eoman 

 numeral X., whence it is called Zygnema deciminiim. This 

 is well represented in PI. xxiii. figs. 3 and 4, of Mr. HassalFs 

 work, while ZijgnewM quininum is figured in PI. xxviii. 

 figs. 1 and 3. 



DESMIDIE^. 



The character of this family is thus given by Mr. Ealfs : 

 — " Eresh-water, figured, mucous, and microscopic Algee, of 

 a green colour; transverse division mostly complete, but in 

 some genera incomplete ; cells or joints of two symmetrical 

 valves, the junction always marked by the division of the 

 endochrome, often also by a constriction ; sporangia formed 



