188 Mr. J. Ralfs on the British Desmidiese. 



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 species. A transverse view, 

 when the two segments are separated, is the best method of 

 ascertaining their number ; the terminal lobe has similar pro- 

 minences. 



The species of Euastr-um are not well defined ; plants of this 

 genus vary greatly in form, and it is not unlikely that young 

 fronds have been described as distinct*. Whenever it is prac- 

 ticable the frond should be examined in four different directions } 

 namely, in the front or usual position, at the side, at the end, 

 and by a transverse or junction view after the segments have 

 separated. 



In this genus Ehrenberg includes Micrasterias, Ag. (not Mi~ 

 crasterias, Ehr.) and Cosmarium ; Meneghini separates the former 

 from it, but includes it in the latter genus. Euastrum appears to 

 me to be distinct from both, and especially from Cosmarium. It 

 agrees with Micrasterias in having lobes and emarginate ends, 

 but the fronds are not incised, nor do the lobes radiate from the 

 centre. From Cosmarium it differs in the lobed and emarginate 

 segments, and also in the inflated projections on the surface. 

 These characters will also distinguish it from the other genera in 

 this family. 



Starch granules have been detected in nearly all the following 

 species by Mr. Jenner and myself. 



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 which separate it from them are deep and directed in- 

 wards and downwards. 



In the second section the fronds are more minute and scarcely 

 visible to the naked eye ; the segments are less decidedly lobed, 

 but the margin is crenate or sinuated ; the terminal portion unites 

 with the basal by a neck-like contraction of the segment, and is 

 therefore never included within a notch ; the corners are rounded. 



* I have, since I wrote the above, found my opinion confirmed by the fol- 

 lowing remarks of Meneghini : — " Lobi quoque, pro setate summopere va- 

 riant, ideoque in characteribus fixandis,formis perfecte evolutis attendendum. 

 Ex hoc factum est quod species nonnullae ab auctoribus ohm distinctae nunc 

 temporis evanuere, nee suspicatio excludi potest circa alias quoque species 

 formas summopere similes sistentes." — Meneghini, ' Synopsis Desmidiearum ' 

 in Linnseal840, p. 218. 



