68 BRITISH DESMIDIE/E. 



Tab. VI. fig. 2. a. portion of a filament ; Ij. portion of a filament with empty 

 joints ; c. empty joint to show the granules ; d. transverse view ; e, f, g. 

 sporangia. 



** Frond simple {binate only when dividing), consisting of two seg- 

 ments united by a suture, where it is most frequently constricted. 



6. MICRASTERIAS, Ag. 



Frond simple, lenticular, deeply divided into two, lobed segments ; the 

 lobes inciso-dentate (rarely only bidentate), and generally radiant. 



The fronds are simple, nearly flat, without inflated protuberances, 

 and divided nearly to the centre, so that the segments are semi-orbi- 

 cular, and usually in close contact with each other along their entire 

 breadth; each is deeply divided into lobes, which are generally 

 arranged in a radiant manner and regularly cleft and dentate at the 

 margin. 



In two species sporangia have been detected ; they are large, glo- 

 bular, and furnished with stout spines, which at first are simple, but 

 finally become branched at the apex. In the perfect state they are 

 particularly interesting from their resemblance to the fossil " Xan- 

 thidia" of Ehrenberg and others. 



The orbicular, plane and deeply incised fronds will distinguish 

 Micrasterias from all other genera in this family. In Euastrum, the 

 only one wdth which it can be confounded, the fronds are oblong and 

 the lobes are not incised. 



As the plants to which Agardh affixed the name of Micrasterias 

 are probably all included under the above description, I have followed 

 Meneghini in retaining that name for this genus, especially as it 

 seems to me that not one of Agardh's species is contained in the 

 Micrasterias of Ehrenberg, who in fact intended by it a very different 

 genus, the Pediastrum of Meyen*. 



Although Micrasterias contains but few species, they admit of an 

 arrangement in three sections. 



The species in the first section present some of the most beautiful 

 microscopic objects amongst the Desmidieae. They are distinctly 



* " Nomen a el. Agardh propositum et perperam ab aliis Pediastris affixum, 

 et cum novo illo Euastrum commutatum servandum." — Menegh. Syn. Besmid. 

 in Lirmcea 1840, p. 215. 



