BRITISH DESMIDIE.li. 69 



visible, appearing to the naked eye like minute green dots. The 

 microscope shows them to be circular and their segments deeply di- 

 vided into five lobes. The end lobe, which is narrowest, is simple, 

 emarginate, and its corners dentate ; the lateral lobes are several times 

 dichotomously incised, the ultimate subdivisions emarginate or biden- 

 tate ; frequently the middle lobes are once more dichotomous than 

 the basal lobes, so as to have twice the number of incisions. The lobes 

 and their subdivisions are alike cuneate, approximate and radiant. 



In the second section the fronds are smaller, and elliptic rather 

 than circular ; the end lobe is somewhat exserted and diverges from 

 the lateral lobes. The division on each side into basal and middle 

 lobes is less marked, so that in some species the segments may more 

 appropriately be called three- than five-lobed. As in the first section 

 the lobes are cuneate and radiant, they are also dichotomously incised, 

 and their ultimate subdivisions are dentate, but fewer in number than 

 in the former, in which there are four to eight in each lateral lobe, 

 M'hilst in this section the ultimate subdivisions in the basal and middle 

 lobes are rarely more than two. 



The third section agrees with the first, inasmuch as its species are 

 also circular, though considerably smaller ; its essential diflference de- 

 pends on the great breadth of the end lobes and the very slight in- , 

 cisions between the lateral ones. 



In the fourth section there is but a single species. It is oblong, 

 its segments are five-lobed, and their lobes slightly emarginate. 



The form of the frond of the species belonging to the fifth section 

 differs from that of the preceding ones. It is not only smaller, 

 but each segment is again constricted, and the direction of the lobes 

 is horizontal. Unlike the lobes in the foregoing sections, these are 

 broadest at the base and attenuated upwards, and are of course di- 

 vergent ; their extremities are simply bidentate. The margin of the 

 end is convex or straight, not emarginate. 



For the reception of one species belonging to this section, Mr. 

 Hassall has constituted his new genus Holocystis ; but, allowing that 

 the plants I have placed here differ in form from the other species in 

 the genus, and giving no opinion on the propriety of their removal, I 

 think it better at present to retain them in Micrasterias, because some 

 of the forms have been regarded by Ehrenberg and Meneghini as 

 merely young states of other species j and Kiitzing, who has described 



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