7.18 SYSTEMATIC HISTOKT OF THE INFUSORIA. 



Closterium attacked by a parasitic growth (?) ; Microtheca, excluded ; Poly- 

 edrium, excluded ; Zygoxanthium = Xanthidium, in part ; Phycastmm, Aste- 

 roxanthium, Stephanoxantliium,=Staurastrum ; Grammatonema, a diatom; 

 Beimhusmei = Didymo2:>r{um Borreri; Isthmosira = Sphaerozosma ; Eucampia, 

 a diatom ; Geminella, excluded ; Raphidium = Ankistrodesmus ; Oocardium, 

 excluded. 



The other genera included by Kiitzing are placed here as a distinct group, 

 Pediastrese. 



Didymocladon {Rolfs) seems not distinguished from certain Staurastra by 

 characteristics sufficient to separate it from them ; we have therefore imited 

 them, in which we follow Brebisson. 



As to the new or altered genera proposed by Nageli and de Baiy, founded 

 rather on the mode of disposition of the endochrome than on the external form, 

 although we do not venture to deny its probably great importance, yet it 

 seems to us that the characters relied on are in many instances not sufficiently 

 constant for the purpose, as well as that several of the known Desmidian 

 species could not be satisfactorily or indubitably referred to the particular 

 genus to which, judging from analogy, they ought to belong ; neither, indeed, 

 does it seem, so far as we can judge, that those writers are themselves satisfied 

 as to the proper place of certain species, nor does the system, as yet, appear 

 quite without the disadvantage of disassociating kindred forms. We believe 

 we are fortified in the opinion we here endeavour to express by that of M. de 

 Brebisson. The genera Cylindrocystis and Mesotsenium are here omitted from 

 this family, as their claims to admission scarcely seem as yet indubitable ; 

 moreover, there seems to us Httle certainty as to the limitation of the species 

 hitherto described by Kiitzing and others. 



If we have omitted some of the species described by the various authors 

 before cited, it is from a conviction that, when either not satisfied as to their 

 absolute distinctness, or unfurnished with what we could look upon as suffi- 

 ciently exact details, it was the safest course we could pui'sue, — as it seemed 

 to us better to leave out a few species, than to insert them with a description 

 which, owing most likely to our own want of perception, might prove insuf- 

 ficient or inaccurate. On the other hand, some may think we have admitted 

 too many species, and that certain of the forms hereafter described may be 

 but "varieties" of whichever may be assumed as the typical specific form; 

 but in this conclusion we cannot coincide, as we are disposed to believe that 

 the species hereafter described (with possibly, indeed, a few rare exceptions) 

 are quite distinct, and, at least so far as British or Irish species are concerned, 

 are always perfectly distinguishable. 



An ingenious method of succinctly expressing by means of symbols the ex- 

 ternal characteristic forms of the genera Tetrachastrum, Micrasterias, and 

 Euastrum, was propounded in a paper by Rev. E. Y. Dixon, read to Nat. Hist. 

 Soc. Dub., 3rd June, 1859. We append his own explanation, as the best that 

 could be given : — 



" The typical mode of division [in the genera above named] (as exemphfied 

 in Euastrum pinnatum, E. ohlongum, &c.) appears to be into three portions or 

 subdivisions, — the first, next the line of separation of the segments, extend- 

 ing across the frond, and embracing the two basal lobes ; the second iucluding 

 the median lobes ; and the third, the extreme or end lobe. This last, or third 

 subdivision, is the most constant. The two former are frequently represented 

 by a mere sinuosity or shallow indentation where the third is distinctly deve- 

 loped ; but we never find the fii'st subdivision distinct, and the second and 

 third imperfectly separated. The whole three, indeed, maybe merely marked 

 by slight sinuosities, as in Euastrum cuneatum ; but if any one is separated, it 



