260 Mr. J. Ralfs on the British Desmidiese. 



mature plant appears more turgid than the rest of the segment ; 

 the other lobes are deeply bifid. All the lobes, as well as the 

 divisions of each lobe, are cuneate ; all the incisions have their 

 sides approximate and point towards the centre of the frond. 

 The margin of the frond is dentated and is frequently colourless. 



The young frond dhTers considerably from the adult form : its 

 terminal lobes are very broad with convex and entire margins, 

 whereas the other portions are dentate or spinulose-dentate at 

 their margins ; the lobes not being yet divided there are no in- 

 cisions, or very short ones, whilst a slight notch often points out 

 the future clefts ; the younger the frond the more entire is its 

 margin. 



As in this plant the portion connecting the two segments is 

 very narrow, the mode of increase by division is well shown. The 

 central connecting portion elongates and becomes converted into 

 two colourless rounded lobules, which increase in size, gradually 

 acquire colour, and finally become lobed and miniature represen- 

 tations of the segments of the frond, and then two new fronds 

 are formed by separation. 



The new segments frequently acquire considerable size before 

 they become lobed. As two fronds are formed by separation be- 

 fore the new segments are fully grown, specimens often occur in 

 which the segments differ in size, and the lobes of the smaller 

 one are sometimes not fully formed. 



Plate VI. fig. 1. Micrasterias rotata : a, full-grown fronds; b, young 

 fronds ; c, mode of increase. 



Micrasterias apiculata, Mngh., which differs from this species 

 in having its surface furnished with spines, has not been detected 

 in this country. 



2. M. Melitensis t Mngh. Frond smooth, all the lobes deeply bifid ; 

 the divisions linear and bidentate at the extremity ; both the lobes 

 and their divisions are divergent. Mngh. I. c. p. 216. Euastrum 

 Crux Melitensis, Ehr. Infus. p. 162. tab. 12. fig. 3 ; Pritch. Infus. 

 p. 196. fig. 124 ; Bailey, /. c. fig. 23 and fig. 29, young. 

 Amongst Diatoma fenestratum in a small pool a little below the 



outlet of Llyn Gwernan near Dolgelley : very rare. 



This, which is a very beautiful plant, is much smaller than 

 Micrasterias rotata ; the segments are deeply divided into five 

 nearly equal lobes which diverge from each other, and are all 

 deeply bipartite ; their divisions are linear, bidentate at the apex 

 and also divergent. 



The colouring matter is bright green and confined to the centre 

 of the frond ; the linear divisions of the lobes being colourless. 



I have not seen the young fronds of this species, but their 



