DUBLIN UNIVERSITY ZOOLOGICAL AND BOTANICAL ASSOCIATION. 237 
elegant monograph,—they are thus defined:—Ere sh-water figured 
mucons and microscopic Algae, of a green colour; transverse division 
mostly complete, but in some genera incomplete; cells or joints of two 
symmetrical halves, the j auction always marked by a division of the 
endochrome, often also by a constriction. Sporangia formed by the 
coupling of the cells and union of their contents.” These are microscopic 
gelatinous organisms, of a green colour, growing in fresh water only, 
the cellulose walls of which are covered with “ pearly granules,” or 
minute puncta, or asperities, or are smooth, without any siliceous coat, 
of peculiar and varied forms, such as oval, lunate, cylindrical, triangular, 
or compressed, &c., usually with variously formed rays or lobes, or quite 
simple, presenting a bilateral symmetry; the junction of the halves 
marked by a division or interruption of the green contents, often, though 
not always, by a constriction of the frond itself; the opposite valves con¬ 
nected by a suture; the individual cells either free, arranged in linear 
series, forming a filament, collected into faggot-like bundles, or dis¬ 
posed in circular, flattened, star-like groups. It will he seen that the 
most striking points of distinction from Diatomacese are the absence of 
the siliceous figured coat, and the contents being green, not brownish or 
yellow. The Desmidiaceae are very striking objects under the microscope, 
from the singularity and variety of their forms, and their curious external 
appendages and lobes. That which is most distinctive and characteristic 
in their appearance is their bilateral symmetry, usually, though not 
always, accompanied by a more or less deep constriction between the 
segments of which each cell is composed. But in these respects there 
exists all shades of difference in the various genera. Tor example, in 
the genus Scenedesmus the symmetrical outline is wanting; in Pedias- 
trum it is indicated by an external notch on the outside only; in Clos- 
terium and Penium, as well as in a new form I have the honour in this 
communication to bring forward, there is no constriction (nevertheless 
the tendency to the bilateral division is indicated in those forms by the 
usual pale hand at the centre of the endochrome) ; in Hyalotheca, va¬ 
rious Cosmaria, Tetmemorus, &c., the constriction is hut slight; in Des- 
midum and Didymoprium it is indicated by only a notch at each angle; 
while in various other genera the constriction becomes deeper and more 
obvious, imtil at last in Sphserozosma, Euastrum, Micrasterias, &c., the 
constricted portion becomes like a mere isthmus between the segments, 
giving them the appearance of distinct cells, and as such they were for¬ 
merly considered. Each frond or individual, however, is always a single 
cell, as is often evidenced by the whole contents escaping through a 
single accidental rupture. The cells frequently possess warty or spi¬ 
nous processes, and the cellulose coat often presents minute markings 
or puncta, caused by the presence of little elevations. The cells are 
usually more or less surrounded by a gelatinous sheath,—in Hyalotheca, 
Didymoprium, and many others, this is well defined, hut in some species 
it is so attenuated as to have its existence made known only by the 
gelatinous investment preventing the contact of the fronds. The con¬ 
tents of the cells of the Desmidiaceae appear to he similar to the green 
vol. v.— proc. soc. 21 
