DUBLIH NATURAL HISTOBY SOCIETY. 205 



By attention to the generic characters as given above, this species can 

 of course bo readily distinguished from every other Desmidian, MicraS' 

 terms oscitans (Ital/s) and M. pinnatifida {Ralfs) excepted. The pre- 

 sence of the marginal mucronate spines and of those terminating the 

 lateral projections of the end lobe, combined with the absence of the 

 incised extremities, 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 Desmidia)" (pages 76, 77) the specific characters of those 

 two species, the first under the name of — 



Dftrachastrum oscitans = Mie. oscitans {Hfs.), Holocystis oscitana 

 {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 Rolfs, they were necessarily introduced as specific dis- 

 tfinctions 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 spe- 

 cies from Tetrachastrum mucronatum, they are not requisite, as the biden- 

 tate 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 pinnattfidum = Ificrasterias pinnatifida {Haifa). 



"Frond plane, its ends straight [segments deeply constricted], 

 lobes [horizontal] triangular, bidentate. 



The same characters which distinguish the preceding species from 

 IHrachastrum mucronatum also separate this, which is moreover much 

 smaller. It appears to differ from the preceding 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 in- 

 volve the necessity of describing these forms as truncate, and without a 

 terminal lobe. The end lobe in these forms is equivalent to the same 

 poi-tion in Micrasterias rotata or M. Crux-Melitenaisy and differs by having 

 more extended lateral projections not divergent, but, as above described, 

 projecting horizontally and parallel in Erection with the attenuated 

 basal lobes. 



The following synopsis of the Schizoraerous Desmidian genera, and 

 of the species of Tetrachastnim will, it is hoped, assist in conveying. 



