81. 



ARCHER AND DIXON, ON DESMID1ACE.E. 



describing these forms as truncate, and without a terminal 

 lobe. The end lobe in these forms is equivalent to the same 

 portion in Micrasterias rotata, or M. Crux-MeUtensis, and 

 differs by having its more extended lateral projections not 

 divergent, but, as above described, projecting horizontally and 

 parallel in direction with the attenuated basal lobes. 



The following synopsis of the Schizomerous Desmidian 

 genera, and of the species of Tetrachastrum, will, it is hoped, 

 assist in conveying, in a succinct manner, the views put 

 forward above, as well as the end sought to be accomplished 

 in the present paper by the institution of the genus. 



r Frond lenticular, as long as or longer 

 than broad ; segments usually semi- 

 orbicular, five- or rarely three-lobed 

 (with three, or rarely two subdivi- 

 sions, 0, b, c, or a^~"b, c) ; lobes 

 radiant, incised or dentate, rarely 

 only sinuate, widening outwards ; 

 central constriction usually linear. 

 Micrasterias. 

 {Vide ' Brit. Desmidieae/ p. 68, 

 et seq.) 

 Frond longer than broad ; segments 

 more or less conical, five- or three- 

 lobed, or sinuated (with three or 

 two subdivisions, either a, b, c, or 

 a^b, c, or ab, c, or rarely ab^c), 

 possessing variously disposed, cir- 

 cular, inflated prominences; ends 

 emarginate, or rarely with merely 

 a concavity ; central constriction 

 linear. Euastntm. 



{Vide ' Brit. Desmidieae/ p. 78, 

 et seg.) 

 Frond about as long as or longer 

 than broad ; segments three-lobed 

 (with two subdivisions, ab, c), 

 without inflated prominences; basal 

 lobes horizontal, attenuated out- 

 wards, end lobe expanded Laterally, 

 its lateral projections parallel in 

 direction with the basal lobes; 

 ends straight or rounded, entire; 

 central constriction forming an 

 acute-angled spreading notch : 



Frond simple, com- 

 pressed, deeply con- 

 stricted, segments 

 lobed or sinuate 

 (Schizomerous) ; 

 lobes either incised, 

 sinuate, or entire. 



