BRITISH DESMIDIE.12. 



Length of frond -j}-^ of an inch ; breadth at basal lobes -ji-j ; breadth at 

 end lobes -j^ ; breadth at constriction between the lobes ysVs • 

 Tab. X. fig. 3. o, b. fronds with endochrome. 



7. EUASTRUM, Ehr. 



Frond simple, compressed, deeply divided into two segments which 

 are emarginate at their ends, lobed or sinuated, generally pyramidal 

 and furnished with circular inflations. 



The fronds are simple, longer than broad, often oblong, compressed, 

 and so deeply constricted that their segments seem only united by a 

 narrow chord. The generally pyramidal segments are broadest at 

 their bases, and are there in such close apposition for their entire 

 breadth as nearly to conceal the notch on each side until the endo- 

 chrome has collapsed. They are attenuated towards the ends, which 

 in the adult state are almost always more or less emarginate, and 

 their sides are more or less lobed or sinuated. The surface is irregular 

 with inflated prominences, which also form tubercle-like projections 

 along the margins ; their number and situation are, probably, constant 

 in the adult fronds of the same species and different in distinct spe- 

 cies. A transverse view is (when the segments are separated) the best 

 method of ascertaining their number ; the terminal lobe has similar 

 prominences. 



In Euastrum, Ehrenberg includes Micrasterias, Ag. (not Micra- 

 steriasy Ehr.) and Cosmarium, and in this he is followed by Kilt zing in 

 his ' Phycologia Germanica.' Meneghini separates Micrasterias from 

 Euastrum, but includes the latter in Cosmarium. Euastrum appears 

 to me distinct from both, and especially from Cosmarium. 



Euastrum agrees with Micrasterias in having lobes and emarginate 

 ends, but the lobes are not incised, nor do they radiate from the 

 centre, and the inflated projections will distinguish it not only from 

 Micrasterias, but from every other genus in the family. From Cos- 

 marium, it differs also in the lobed and emarginate segments. 



I have divided this genus into three sections. In the first section 

 the fronds are comparatively large, and appear to the naked eye like 

 roundish or oblong dots. The segments are distinctly lobed; the 

 terminal lobe, cuneate and itself emarginate, is partly included in a 

 notch between the projections of the lateral lobes, and the sinuses 



