212 DUBLIN NATURAL HI8T0ET SOCIETY. 



undulated outline; in fact, a cast from the new segments. In this 

 species the segments appear to me to possess one or two slight basal 

 inflations, though described as with only one in " British Desmidiae." I 

 believe it, however, to be quite distinct from D. Ehrenlergii. — Whether 

 a hyaline pellicle-like investment, sometimes met with, entirely sur- 

 rounding certain single individuals, and apparently like a loose tunic, 

 is anything analogous to the above-mentioned production, I cannot pre- 

 tend to say ; but I have occasionally seen such in some species, — for 

 example, Euastrum didelta. This involving cyst does not follow the 

 boundary of the form, but is of a rounded or oval outline, and generally, 

 when I have noticed it, the contained frond seemed to have lost vitality, 

 the endochrome being brownish. What I allude to is not to be mistaken 

 for the gelatinous investment surrounding the frond in so many Desmi- 

 dian species, being altogether different from anything of the sort seen 

 in fresh specimens. Possibly, like the former, it may be due to its con- 

 solidation — here producing, by a process of superficial condensation, as 

 it were, a kind of skin, from beneath which the intervening gelatinous 

 substance has been absorbed. I hazard such a conjecture the more, be- 

 cause something very like this seems actually to occur in some species 

 of Gloeocapsa, or allied forms, in which the concentric gelatinous layers 

 seem to harden into so many frangible investments. 



In the foregoing very brief account of the mode of ceU-di vision 

 which occurs in this family, it will be noticed that I look upon the 

 formation of a septam at the isthmus as the preliminary or initial step 

 in the process. Now it appears to me that the accompanying figures 

 represent individuals, which, having taken on them the vegetative 

 growth, or effort to repeat themselves by transverse division, through 

 some inexplicable cause have omitted the formation of a septum. This 

 not having taken place simultaneously with the vegetative activity being 

 aroused, which, being in full energy, proceeded to the development of 

 new growth, which, according to the law which prevails in this family, 

 took place, as usual, between the older segments, in consequence, there- 

 fore, the resulting formation in each case consisted of but one cavity, 

 and fresh endochrome being formed, they each became entirely fiUed. 

 Nor do I think some instances lately under observation, and to which I 

 will just allude, are a contradiction to this. Specimens of Penium 

 cylindrus, to all appearance perfectly healthy, and manifestly under- 

 going growth, lately occurred to me ; of these, a few individuals pre- 

 sented themselves, in which various stages of the new growth, produced 

 in the usual manner in this species on a line with the older segments, 

 had been accomplished, in some cases the fronds having attained to 

 double the ordinary length — but in none of the instances referred to 

 was any appearance of a septum evident. That the fronds had added 

 to their length by recent new growth was proved by its usual colourless 

 cell- wall, as compared vn\h. the red-tinted older segments. That there 

 was no septum was proved by the granular particles partaking of a cir- 

 culatory motion at and past the central point ; and indeed, when present, 

 it is readily seen as a transverse line. No further alteration took place 



