1078 The American Naturalist. [December, 
present the following statement, which holds good for all hard-_ 
shelled Rhizopods : 
The shell of fresh-water Rhizopods is composed of two elements : 
(a) Silica, always in the form of detached pieces, and forming 
most of the mass of the shell; (4) chitinoid, or chitino-siliceous 
matter, serving as a substratum or soldering magma. 
(a) SILICA. 
Silica is first found represented by fine particles of sand. In 
this state it generally constitutes nearly the whole of the shell in 
the genus Difflugia, especially in the species of this genus which 
frequent the bottoms of rivers or clear-water ponds. In these lat- 
ter species the amount of chitinoid matter is so small as to be 
scarcely discernible, and the shells, when compressed, show 
hardly any elasticity, their various elements or sand particles being 
easily disaggregated. 
Besides being found in Difflugias sensu stricto (iooi thereby 
those species of the genus whose shells are normally built of 
quartz grains), silica in the state of sand particles can be found 
in many Rhizopods (Difflugia in part, Centropyxis, Heleopera, 
etc.), but in these cases generally forms a part only, and not an 
important one, in the constitution of the shell. 
I must mention here a very curious fact, to which I called 
attention in 1880, and which the observations I have since made 
at Geneva on a new interesting species have shown me to be of 
more frequent occurrence than I at thatstime thought; namely, 
that in certain species (Diff. lucida, Diff. fallax) the shell, very 
much like that of one built up of true sand particles, is in reality 
covered on its entire surface with amorphous siliceous elements, 
transparent, colorless, often rather flattened, less angular than real 
stones, a product of the animal itself, and constituting in these 
species a remarkable case of mimicry. 
Very often, and in numerous species, silica is to be found as 
amorphous plates or scales, seldom alone (Heleopera rosea), more 
often mixed up with sand grains or othér elements (diatoms). 
Diatoms are very frequent in a great number of species (Difflugia — 
i.p., Pseudodifflugia, Centropyxis, Lecquereusia, Nebela, etc.). It 

