DUBLIN UNIVEESITY ZOOLOGICAL AND BOTANICAL ASSOCIATION. 255 
I was not able to see its commencement, bnt in the specimen from which 
the drawing has been taken, in about thirty-six hours from the time I 
saw the stage represented in the figure (Fig. 15), the new halves were 
completed. As is usual, the new segments maintained a connexion with 
each other simply by the extremity, until they had nearly fully grown, 
when they became detached; but I could not see that they were held 
together, or at all surrounded by any gelatinous investment. Like Pe¬ 
nium Brebissonii and others, however, it may be that this is sometimes 
absent, though at other times abundantly evident. I apprehend that 
the act of division indicates that the individual had attained the full 
form and size of the species. The straight outline of this form, as, 
indeed, I need hardly point out, at once excludes it from Closterium, 
while it is, of course, equally decisively distinguished from Docidium 
by its tapering form, rounded (not truncate) ends, and by its want of a 
central constriction and its non-inflated segments;—from Spirotaenia 
(in the recent state at least), its scattered, non-spiral endochrome at 
once removes it;—the want of a constriction and terminal notch excludes 
it from Tetmemorus;—while, from all the foregoing circumstances, it 
will be seen that it is really a Penium. From the striate, or granulate 
species of Penium, it may at once be known by its smooth frond, from 
which, indeed, its attenuated ends would sufficiently distinguish it. Of 
the species with smooth fronds, it appears to have greatest affinity to 
Penium closterioides (Rolfs ), by reason of its terminal cavities with mov¬ 
ing granules, and its fusiform outline. But, in the form in question, its 
very much smaller dimensions, combined with the entire absence of, or 
sometimes faintly apparent, longitudinal fillets, as well as the presence 
of only two conspicuous dense corpuscles (not a longitudinal series), its 
more cuneate segments, and its more narrow and slender ends, to which 
the terminal cavities are closer, readily distinguish it from that species. 
From the other smooth species, except Penium interruptum ( Breb .), the 
presence of the terminal cavities, containing active granules, at once 
removes it, while it never could be mistaken for that species (with which 
it is, indeed, unnecessary to compare it), on account of its far more 
minute size, as well as the absence of the two additional transverse 
bands, and its fusiform (not cylindrical) outline. Neither can it be 
mistaken for Cosmarium curium ( Rolfs ) = Penium eurtum ( Breb .), from 
which it is at once separated by its smaller size, less inflated appearance, 
the want of any central constriction and of so conspicuous longitudinal 
fillets, as well as by the possession of the terminal cavities, containing 
moving granules. 
There is only one other form with which I need particularly contrast 
it, and that is Spirotcenia obscura (Rolfs ). At first sight it might appear 
unlikely to be mistaken for that species, nor is there any resemblance 
when fresh specimens of both are examined. Then the spiral arrange¬ 
ment of the endochrome in Spirotsenia alone is an abundantly sufficient 
mark of distinction; but when Spirotcenia obscura is kept for some time 
in the house, this spiral disposition of the endochrome is lost, and it be¬ 
comes uniformly green. Moreover, there not unfrequently occurs in 
