
* 

1891.] Rocky Mountain Rhizopods. 1081 
disperses them so widely in every direction that it is generally 
impossible to find them again. Yet, if one takes the precaution 
to isolate a shell (say Euglypha) in a very small drop of acid, 
one finds after the action of the boiling acid all the plates again; 
but they are dissociated from each other, and in a little heap. In 
this case the chitinoid matter is gone, and has left only the pure 
siliceous plates. 
At the same time, it must be added that in some species the 
cementing ‘matter seems to be purely chitinoid in some regions 
of the shell ; for instance, about the mouth in Sphenoderia dentata; 
Corythium pulchellum, and others. 
There are two genera of which I have not yet spoken, and 
concerning which I should like to say a few words,—namely, 
Cyphoderia and Arcella. 
The shell of Cyphoderia, with its elegant covering of small, 
regular, hexagonal alveoli, is very currently considered to be made 
up, first, of an internal, brownish, chitinous pellicle, and then of 
an external envelope, itself consisting of hexagonal, chitinous 
prisms. The experiments I made at Geneva on this species have 
shown me that it is not so. In reality, the internal chitinoid pel- 
licle is covered over its whole surface with small discs, or rather 
cylinders, consisting of pure silica. These I was able to isolate 
and examine on all their faces. They are circular in section, 
about one-third larger in diameter than in height, flat, or very 
little excavated on their upper and lower faces, and have altogether 
the form of fish vertebrae. Their diameter is in the bigger shells 
(var. major) about 0.002 mm. ; their size is uniform over the whole 
shell. They are disposed with a wonderful regularity, touching 
each other by the borders, and cemented together by the chitinoid 
matter which penetrates into the interstices and often flows out to 
the outside. The appearance of hexagonal alveoli is a result of 
the juxtaposition of all these small cylinders and of the mepo 
tion of the cementing matter. 
As for the shell in Arcella, I feel confident that it is analogous 
to that of Cyphoderia ; but the siliceous elements are very much 
smaller, and the experiments I have made have not been decisive. 
Yet I have seen on broken shells that the lines of fracture were 
kd 

