52 OF THE DESMIDIACEjE AND SIMILAR MINUTE ALGAS. 
In some instances, the individual is constricted in such a 
manner, as to have the appearance of two valves united 
together centrally : of these, fig. 58, Plate xii., is an example. 
The reader will observe the same peculiarity of structure 
in the figures ranging from 17 to 22, already given in Plates 
iv. v. Other species, again, are arranged in long filaments ; 
or ribbon-like colonies, fig. 59, Plate xiii. 
These filamentous Desmids generally yield very clean 
preparations. 
It will be seen that Hyalotheca is enveloped in a thick 
coating of mucous matter : this is by no means a common 
form of construction, though we find instances here and 
there, of which Spirotamia condensata, Plate xiii., fig 60, is 
an example. 
The decayed remains of Sphagnum in peat bogs are 
often rich with the empty trustifies, well adapted for ex- 
amination, though, from their dirty brown colour, and from 
being mingled with the surrounding vegetable matter, they 
are not fit specimens for the herbarium. Having lost the 
bright green of the living Desmid, they are difficult to dis- 
tinguish with the naked eye ; in fact, they are mere empty 
cases ; but none the less welcome to the collector on that 
account, as they often give him opportunities of studying 
the cell membrane, which he cannot get with specimens 
better adapted for preservation. And here let me recom- 
mend him to be always provided with plenty of phials on 
his excursions, that each gathering may be kept apart; 
otherwise he entails upon himself a vast amount of trouble, 
when he comes to separate the species for preparation. 
Many of the Desmidiacese will readily propagate when 
kept in a room in a sufficient quantity of bog-water, thus 
enabling one to get an abundant supply of clean specimens. 
The plate, or saucer, in which they are placed, should be 
covered with a sheet of glass to keep out the dust, and to 
prevent the water from evaporating too rapidly. However, 
should the water get too low from this or any other cause, 
the supply must be made up with rain or soft water, on no 
