304 
Proceedings of the Royal Irish Academy. 
to the foregoing. The most ready manner to convey an idea of the 
figare of this also very minute form is, to say it resembles that of an 
Arthrodesmus ; in other words, it is compressed, quadrangular, divided 
by two opposite, deep, wide, triangular notches into two broad cuneate 
segments, their lower (or inner) sides slightly convex, and the outer 
angle subacute, and very minutely apiculate, and the upper (or 
outer) sides very slightly concave at the middle, somewhat raised 
towards the angles,"^' the contents pale seruginous colour, and 
homogeneous looking. This form occurs of different sizes, some 
of them being twice the linear measurements of others, but, as mentioned, 
even the largest is exceedingly minute. Though thus arthrodesmoid 
in figure, apart from the colour and a general aspect of contents, the 
greatly differing sizes occurring in one and the same gathering, would 
prevent this being taken at all for any desmid ; neither in this nor the 
following form is any gelatinous envelope apparent. A certain amount 
of a kind of movement shown by this form is, perhaps, not at all 
special, yet it is always somewhat strikingly evinced. I allude to a 
gentle jerking or oscillatory movement of each cell — a sort of quiet from 
side- to-side vibration, generally, as if on a pivot running through the 
isthmus, the radius of action being but restricted, yet constant. Owing 
to this gentle movement not being quite equable, in fact rather irregular, 
a change of position of the cells occurs, but no noteworthy or decided 
change of place. I would not be disposed to attribute any great im- 
portance to this, yet it is a minor phenomenon, which, in this little 
form, when met with, hardly ever fails to arrest one's attention. This 
must, I should think, be accounted a rare form, though it casually 
turns up in several places, especially from county Dublin and county 
Wicklow. I have not been able to see any new growth or formation of 
younger segments, nor any noteworthy difference in the examples now 
brief) visit of his to tMs country —an occasion of whicli I must ever retain a lively 
and pleasing recollection. We spoke, inter alia, of these forms, the subject arising 
from his having shown me a sketch of a minute form found by him in Sweden, 
which I at once recognised as most probably identical with that above recorded as 
Tretrapedia Remschiana. I had fortunately, ere long, an opportunity to show Dr. 
Wittrock a fresh example not only of this, but likewise of the other form now here 
named Tetrapedia setigera (we were at the moment in the far West, in the middle of 
Connemara's wilds), and I hope that skilled and accomplished observer may not consi- 
der it too great a liberty on my part to mention that, whilst he identified the former 
as occurring in his country, he likewise concurred in my view, above expressed in 
regard to both, as to their nature and affinity. 
* I regret that the figures in the plate (figs. 11 and 12) do not sufficiently accu- 
rately indicate the characteristic sought to be described by the words, " somewhat 
raised towards the angles;" the figures show the upper margin of the segment too 
uniformly concave, whereas each lateral portion or lobe of the segment presents 
rather a mamillate outline, that is the upper margin (as well as the lower) slightly 
convex towards the angles, but concave at the middle. It will be, of course, imdcr- 
stood that the two figures (11 and 12) each show an individual, one with the 
*' segments" lying horizontally, the other vertically, the deep sinus being that 
making the bilateral ity, or arthrodesmoid contour. 
