ane? See T= 
201 
to detect their presence at once. In some three or four species, 
Mr. Thwaites and Professor Smith are reported to have seen the 
contained granules in active motion. I have never been able to 
detect this action in the species named by these gentlemen ; but 
I have been fortunate enough to witness it in two other species I 
gathered in this neighbourhood.* 
In the size and shape of the diatom frustule there is an almost 
infinite diversity. Not a few are very narrow, and much elong- 
ated (as Synedra, etc.), and, from presenting a wand-like figure, 
suggested to Ehrenberg the name of Bacillaria to designate the 
family. Some, again, are triangular ( Zriceriatum), or square 
(Ampheteteras ), or oblong ( Zerpsene). Others, again, grow in 
filaments, and, when separated, present the appearance of tiny 
pill-boxes and globes, owing to the convexity of their valves 
(Melosira). Several genera are boat-shaped (/Vavicula ), some are 
oval (Surerelia). Many—and these the most beautiful—are 
flattened discs ( Cocinodiscus, Heleopelta, Arachnodiscus ); while not 
a few are wedge-shaped (Licmophera). Some are curved and 
twisted upon themselves, and others assume a sigmoid form 
( Pleurosigma ), or an undulated form (as Symatapleura). A con- 
siderable number of species are arcuate (Zunolia ), saddle-shaped 
( Campylodiscus), and one genus has every species shaped like 
plugs ( Gomphonema ). 
Nor are the conditions under which the Diatomacez are 
found to exist much less diversified than their form. For con- 
venience of classification, however, they have been represented as 
existing under four general conditions, viz :— 
ist, Free; the frustule having undergone perfect, or entire, 
subdivision, are found in a free or unconnected state (WVavicula ). 
2nd, Stipitate; the frustules being implanted on a common 
stem, which keeps them in mutual connection after they themselves 
* The first time I saw the action was in some specimens of Pinularia 
viridis (Smith) obtained on the Rifle-range, near Whitehaven, in December, 
1876. The other instance was furnished by some specimens of Pleurosigma 
angulatum (Smith) collected in the Harbour the following autumn,—ScIENCE 
Gossip, Vol, IV. 
