
1891.] Rocky Mountain Rhizopods. 1079 
is very seldom that they constitute normally the total covering of 
the shell (Diff. bacillifera, Diff. rubescens); yet, even in species 
where as a rule these algæ only make up a part of the envelope, 
one may always expect to find isolated specimens where they 
constitute the total covering. 
All the siliceous elements which I have until now mentioned 
are irregular, either in their form or in their relative sizes on the 
shell ; but there exist a whole series of genera where these elements 
are conspicuous by their geometrically regular form. However, 
before speaking of these I shall mention some forms which 
occupy a somewhat intermediate position. They belong to the 
family of Nebelidz. In this family the siliceous elements are 
represented by regular circular or oval discs, contiguous with each 
other and covering the whole shell. Sometimes all these discs 
are very nearly of the same size all over the shell, at other times 
large ones are mixed up with very much smaller ones. These 
discs are generally very ,but in the genus Hyalosphenia 
and in some Nebela they cannot be seen, and the shell looks as 
if it were composed of a continuous chitinoid membrane. Yet 
in these species it is most probable that the discs really exist, but 
are very thin, and hidden in the abundant chitino-siliceous matter 
of the envelope. 
We now come to those genera in which the siliceous scales 
have regular forms and are symmetrically disposed. The genus 
Quadrula is remarkable for its square, colorless scales, touching 
each other with their borders and arranged in regular series. In 
the numerous species of Euglypha the plates are oval, sometimes 
circular, seldom cordiform, but always perfect in their shape, 
disposed in diagonal series over the whole shell, and slightly 
Overlapping each other. The circular or oval plates of Spheno- 
deria, Trinema, Placocysta, are also perfect; but in some species, 
owing to the form of the shell, they may vary very much in 
size in different regions of the shell. In the Assudina semilunum 
the plates are elongated, very thick, and often incline to a slight 
asymmetry. In Corythium dubium they are still all alike, but 


have more the shape of an elongated rectangle. 

