264 NATURAL HISTOKY SOCIETY OF DUBLIN. 



genera. In order to illustrate this, indeed, I need but refer to the 

 old multifarious genera, Conferva and Lichen. Because the incon- 

 gruous species formerly included in these old, and as we now know 

 unnaturally comprehensive, genera, have had to be parcelled out here 

 and there according to the special characteristics and affinities of each, 

 notwithstanding blanks in our knowledge as regards several of the 

 smaller groups, it has not happened that this task has been in many 

 respects, or on the whole, quite a hopeless one. JSTor do I think, so 

 far as I can see, that the breaking up of the species of the genus Pal- 

 mogloea which fall under Cylindrocystis, Mesotaenium, and Spirotsenia, 

 has been quite unsuccessful ; nor can I see how the advisability of the 

 step can in itself be adduced as an argument as to the uncertainty of 

 those forms, but should rather regard it as a natural consequence of 

 their characters being happily better established ; the others formerly 

 included in Palmoglcea have to be carried elsewhere — one I think, at 

 least, to Chroococcaceae ; and their true nature, I quite admit, does not 

 seem at all so well established. 



Further, Dr. Hicks goes on to remark : — " If by one observer the en- 

 velope of mucoid matter be taken as a specific or even generic sign — if 

 the mode of segmentation be taken by another as of specific or generic 

 value — if the size of the cell, or the position of the nucleus, or the mode 

 of diffusion of the endochrome within the cell, be sufficient in the eyes 

 of another to separate genera — if, as Mr. Archer contends, the oval 

 shape is another important distinction — it seems to me no wonder that 

 the difficulty acknowledged by all has arisen." These remarks are in- 

 tended to be applied to the genus Palmoglcea ; and I quite acquiesce 

 with their author, that no wonder the difficulty adverted to should 

 have arisen, when each single observer pays attention to one only of 

 such characteristics, disregarding all the rest. The genera Cylindro- 

 cystis, Mesotaenium, and Spirotsenia (which are those in question), each, 

 it is true, possesses an envelope of mucoid matter, but it indeed exists 

 also in many Palmellaceo3, as well as Chroococcaceae/likewise in Desmi- 

 diacese, &c. The transverse mode of segmentation — that is, through 

 the shorter diameter — occurring in these genera, takes place also in 

 Desmidiacese, in the elongate forms ofPalmellaceae, as well as of Chroo- 

 coccaceae. The oval shape, too, is shared by the forms in question with 

 several other forms in both those families. The size of the cell like- 

 wise varies in these plants, within certain limits, in the same species. 

 The nucleus is very hard to be made out — indeed, I doubt if it is always 

 to be perceived or existent even in Palmellacese. Nevertheless, I hold 

 that the forms in question are abundantly distinct; and that, when the 

 eye becomes familiarized with them, they can be at once recognised. It 

 is, of course, here as elsewhere, on the special characters possessed in 

 common by certain groups of the species, combined with certain of the 

 foregoing general characters, and not upon any one or more of 

 the general characters, as suggested by Dr. Hicks, that we must rely 

 as of generic value, and on the ultimate individual proper charac- 



