86 PROCEEDINGS OF SOCIETIES, 
the form described by Dr. Greville, the central blank interspace 
seems from the figure to be smooth, traversed by a longitudinal 
median line, interrupted at the centre by a well-defined but small 
nodule. In P. plena an elevated siliceous band traverses longi- 
tudinally the blank interspace, imbedded in which the central 
median line may be traced from the extremities towards a central 
nodule of very large dimensions. Mr. O’Meara proposed the 
specific name plena for this form, as it seemed suitable for the 
purpose of denoting the affinity between it and P. semiplena, as 
well as descriptive of its characteristic differences. 
Mr. Archer exhibited a plant collected by Dr. E. Perceval 
Wright on a recent visit to the Arran Isles. Although Mr. 
Archer could not see any very solid distinction between the 
genera Hydrocoleum (Kiitz.) and Cthonoblastus (Kiitz.) = Micro- 
coleus (Hary.), yet, as regards the identification of the present 
form now shown, there did not seem any tangible differences 
between it and Hydrocoleum thermale (Kiitz.). This occurred 
mixed with a number of other oscillatoriaceous plants, forming a 
dense felty coating on rocks. The plant itself formed groups of 
Oscillatoria-like filaments included within a hyaline sheath; in 
fact, agreeing completely with the form named Hydrocolewm 
thermale. But Mr. Archer’s object, in now drawing attention to 
it, was to note a curious modification of the oscillatoriaceous 
movement evinced by these filaments. As is well known, the 
movement of a single free filament of an Oscillatoria consists of 
a vibration or spiral twisting, whereas the movement of these 
filaments, confined in the common tube, consisted of a gliding up 
and down past one another within the tube or sheath. At the 
central portion of the sheath the filaments appeared so closely in 
contact that their outlines were not very distinguishable, and the 
individual motion of each filament, now up, now down, lent a very 
curious appearance, deceptively like a circulation of contents in a 
longitudinal direction. That it was, however, really a gliding up 
and down of the filaments themselves, was abundantly proved by 
looking at the place where the filaments projected beyond the 
opening of the sheath. Here the filaments were seen slowly 
altering the relative proportion of each, which, at any particular 
time, extended beyond the opening of the sheath ; so that, in this 
respect, the aspect of the tuft of projecting filaments was slowly 
but constantly changing.’ The filaments at the free end displayed 
little or no oscillatory movement, their efforts being confined to 
the back-and-forward motion in and out of the apex of the sheath, 
which itself presented a more or less broken and indefinite out- 
line. This kind of movement of oscillatoriaceous filaments seemed 
sufficiently marked in the present instance to deserve this brief 
mention. In touching on the Oscillatoriacee, Mr. Archer thought 
it might not be out of place to exhibit Musset’s paper and figures 
(‘ Nouvelles Recherches anatomiques et physiologiques sur les 
Oscillaires’), of which he had recently become possessed, in which 
— 
