Mr. J. Ralfs on the British Desmidiese. 259 



Micrasterias, Ag. (Meneghini). 



Fronds simple, plane, orbicular, deeply divided into two seg- 

 ments, each of which is lobed ; the lobes are radiant and in- 

 ciso-serrate. 



The fronds are simple, circular, flat, divided nearly to the centre, 

 so that the segments are united only by a narrow chord. The 

 segments are semiorbicular and in close contact with each other 

 along their entire breadth ; each is deeply divided into lobes which 

 are arranged in a radiant manner ; each lobe is regularly and 

 deeply cleft, and the margin is dentate. 



The orbicular, plane, and deeply incised fronds will distinguish 

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

 only one with which it can be confounded, the fronds are gene- 

 rally oblong, and the lobes are not incised. 



Starch granules are very evident in this genus. Young fronds 

 differ so much in appearance from the full-grown plant, that 

 they may be mistaken for another species. 



As the plants to which Agardh affixed the name of Micraste- 

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

 followed Meneghini in retaining that name for this genus, espe- 

 cially as it seems to me that not one of Agardh' s species is con- 

 tained in the Micrasterias of Ehrenberg, who intended by it a 

 very different genus, the Pediastrum of Meyen*. 



1. M. rotata. Fronds smooth; lobes broadly cuneate, approximate, 

 the end lobe emarginate, the others deeply incised, the divisions 

 notched and dentate. Micrasterias rota, Mngh. Syn. Desmid. in 

 Linnsea 1840, p. 215. Echinella rotata, Grev. in Hook. Br. Fl. 

 vol. ii. p. 398 (1830). Eutomia rotata, Harv. Br. Alg. p. 187. 

 Euastrum rota, Ehr. Infus. p. 167. tab. 12. fig. 1 ; Pritch. Infus. 

 p. 195. figs. 121 to 123 ; Bailey, American Bacillaria, pi. 1. fig. 22 

 and fig. 24. Micrasterias, Ag. Bot. Zeit. 1827? 

 In freshwater pools, probably common. Sussex, near South- 

 ampton, &c, Mr. Jenner ; Henfield, Mr. Borrer ; frequent about 

 Barmouth, Rev. T. Salwey ; Dolgelley, Cwm Bychan, Carnarvon ; 

 Carmarthen, Swansea ; Dartmoor and Penzance. 



This plant is not uncommon, and frequently occurs in consi- 

 derable quantities, and unmixed with other algae. 



The fronds are large, and appear to the naked eye like minute 

 green dots ; they are nearly circular ; each segment is deeply di- 

 vided into five lobes. The end lobe is the narrowest, and in the 



* " Nomen a cl. Agardh propositum et perperam ab aliis Pediastris affix- 

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

 Desmid. in Linnsea 1840, p. 215. 



