advanced a certain distance will, without any discoverable cause, suddenly reverse the direction and retrograde. It is a very interesting sight sometimes to watch these minute organisms in their advance across the field of the microscope, I have seen in O. tenuis the two or three end cells waving backwards and forwards slowly and deliberately as though the filament were feeling its way across the field of view. On meeting an obstacle, such as another Alga, the filament would halt as though it were investigating the nature of the obstruction—if the Alga happened to lie at an acute angle the Oscillatoria would accommodate itself and move along the side in contact with it; if, however, it lay at a right angle, it would, after a short examination, pass either over or under it, and continue its onward march, or occasionally begin to move backwards, and so retrace its steps. In one specimen the terminal cell was surmounted by a short bristle, which was used apparently as a feeler. The singular deliberative motions I have attempted to describe, I have never seen except when the filament was moving forward; they do not seem to occur when it is performing a movement of retrogression.
Oscillatoria, sp. n.? The usual method of reproduction in this genus is stated to be by the breaking up of the filaments, each articulation of which then takes upon itself the functions of a gonidium. I believe, however, they do sometimes emit spores, though I have not had an opportunity of watching them after their detachment from the parent filament. And in one instance—a specimen occurring as a purple stratum on a damp stone, which I have not been able to specify—there appears to be a series of special sporiferous cells, amongst, but quite distinct from, the ordinary articulation. (Fig. 5.)
Lyngbya, common.
Polypothrix distorta (?). I do not feel sure of this species, as I have seen only a single specimen.
Nostochaæ.
Nostoc commune, M. Dic., 4. {{{1}}}„ verrucosum, H., lxxvi.
Ulvaceæ.
Enteromorpha intestinalis, H., lxxvii; M. Dic., 5.
Ulva bullosa, H., lxxviii.
{{{1}}}„ crispa, H., lxxviii.
Tetraspora (lubrica?), H., lxxviii.
Palmellaceæ.
Microhaloa rupestris, M. Dic., 3.
Botrydina vulgaris, H., lxxxi.
Coccochloris vulgaris, M. Dic., 3.
{{{1}}}„ protuberans, H., lxxvi.
