ARCHER AND DIXON, ON DESMIDIACEiE. 83 



By attention to the generic characters as given above, this 

 species can of course be readily distinguished from every other 

 Desmidian, Micrasterias oscitans, Ralfs, and M. pinnatifida, 

 Ralfs, excepted. The presence of the marginal mucronate 

 spines, and of those terminating the lateral projections of the 

 end lobe, combined with the absence of the incised extre- 

 mities, as well as the frond being longer than broad, at once 

 distinguish this from both those species, which, as a matter of 

 course, I here include in this genus. It may be advisable 

 here to transcribe from ' The British Desmidieae ; (pages 76, 

 77) the specific characters of both those two species, the 

 first under the name of — 



Tetrachastrum oscitans = Mic. oscitans, Ralfs, Holocystis 

 oscitans, Hassall. 



"Frond with convex ends [segments constricted], lobes 

 [horizontal] conical, bidentate." 



The characters here placed between brackets become 

 generic by transferring this species to this new genus, but as 

 it was included in Micrasterias by Ralfs, they were necessarily 

 introduced as specific distinctions from the proper species of 

 that genus. From the remarks in the preceding part of this 

 paper on the new genus, it will, I hope, be admitted that 

 they are really generic. In order to distinguish this species 

 from Tetrachastrum mucronatum, they are not requisite, as 

 the bidentate extremities to the lobes, with the absence of 

 the mucros, and the frond being nearly about as broad as 

 long, readily do so. 



The remaining species will be — 



Tetrachastrum pinnatifidum = Micrasterias pinnatifida, 

 Ralfs. 



" Frond plane, its ends straight [segments deeply con- 

 stricted], lobes [horizontal] triangular, bidentate." 



The same characters which distinguish the preceding species 

 from Tetrachastrum mucronatum also separate this, which is 

 moreover much smaller. It appears to differ from the pre- 

 ceding by its much smaller size, straight or slightly concave 

 ends, more tapering lobes, and paler colour. 



There can be no doubt, it is imagined, that the view taken 

 above is correct in defining this genus as three-lobed, that is, 

 with two basal lobes and a laterally expanded terminal lobe, 

 and not four-lobed, that is, counting the lateral projections of 

 the end lobe as two, which would involve the necessity of 



