288 
merly growing and expose it to the sun, and it will die in a 
few hours and become quite bleached — a fact clearly showing 
that a constant supply of pure cold water is essential for its 
existence. 
Surirella biseriata, Navicula nobilis, and, indeed, many 
other species, prefer the pure spring water of boggy pools on 
exposed hill-sides. All, indeed, require for their abundant 
propagation pure water and sunshine, with a certain degree 
of warmth. Consequently, stagnant pools, overshadowed by 
bushes and the boughs of trees, are never productive of 
diatoms, the only species I have ever found alive in water 
after it had actually become quite putrid being Cymato- 
pleura solea and C. elliptica ; but how long even they could 
have continued to exist under such conditions I have not 
ascertained. 
Shallow, rapid rivers and mountain streams, and the rills 
and springs of pure water supplying them, are the favorite 
haunts of a very large number of species. During a con- 
tinuance of dry, warm weather in spring, summer, and 
autumn, they are propagated in surprising abundance on 
the sandy or gravelly bottoms of gently running pools, or in 
the quiet eddies by the margins even of rapid streams, or of 
shallow pools distant from the main stream, and subject to 
the inundation of floods, at other times supplied with water 
percolating through the sand or gravel. In all these locali- 
ties a plentiful supply may be obtained by scooping up the 
coloured stratum at the bottom with a spoon, and separating 
the sand from the diatoms by subsidence, and, if a sandy 
instead of a muddy portion of the bottom is selected, a 
gathering will be secured unmixed with impurities, and 
exceedingly rich. In the daytime, especially in sunny 
weather, the stratum of diatoms at the bottom is floated, by 
the air bubbles generated and entangled, to the surface in 
broken masses or flakes, which may be collected from the 
surface by a spoon, and placed in bottles. These flakes 
floating on the surface being quite buoyant, are wafted by 
the breezes into the main current, and are carried downwards 
by the stream : so that there is no better method of securing 
a supply of a great variety of species than walking by the 
side of a river and collecting the brown, jelly-looking flakes 
as they float onwards on its surface, to be finally deposited in 
the mud of its estuary, from which some of the species of 
which it is composed may afterwards chance to be collected, 
and described as brackish water forms. 
The species forming the subject of the present memoir 
were discovered in localities such as I have indicated, and in 
