Structure of Rhizopod Shells. 311 
sideration as being employed by the Rhizopoda in the 
construction of their shell are of threefold nature*. A 
part of the Thalamophora construct their shells of 
agelutinated foreign bodies, partly inorganic (sand, mud), 
partly of organic nature (Thalamophoran and Radiolarian 
shells, sponge-spicules, &c.), while the greater part of the 
Thalamophoran shells are formed by secretion of carbonate 
of lime ; and, thirdly, the skeletons of the Radiolaria consist 
of silica. The two first-mentioned materials of the Tha- 
lamophoran shells have this in common, that they possess 
far less firmness than the silicic acid of the Radiolaria. 
This distinction has also as its consequence a corresponding 
difference in the habit and mode of construction of the two 
great primary groups of the Rhizopoda. 
Even upon a superficial examination one is struck with the 
fact that the shells of the Thalamophora with much less 
multiplicity of form and differentiation are far more massive 
and stouter than the Radiolarian skeletons, which are often 
exceedingly complicated, graceful, and elegant. The com- 
paratively soft material which is employed by the Thalamo- 
phora in the construction of their shells does not permit these 
Rhizopoda without injuring the stability of their dwellings to 
make such airy and complicated structures as the Radiolarian 
skeletons, composed of solid more or less elastic siliceous 
rods. 
The distinctions, however, are of a still more profound 
nature, and extend not only to the external habit, but also to 
the whole structural plan of the shells and skeletons. Even 
in the single-shelled forms, this, as already indicated, may be 
distinctly recognized in the selection or distribution of the 
form-types in the two great sister-groups of the Rhizopoda. 
The monothalamous Thalamophorous shells are almost all 
pylomatic, and only a few forms, such as Orbulinella, Orbu- 
lina, and some sandy-shelled forms, belong to the perforate 
form-type. On the other hand, among the Radiolaria the 
* The primary chitinous shell of many freshwater Rhizopoda plays too 
subordinate a part in the matters here under consideration to need any 
special mention. The shell-material of the Pheodarian families Circopo- 
rida, Tuscarorida, and Challengerida requires closer investigation. It 
appears, however, to have a similar consistency to the calcareous material 
ot the Thalamophoran shells, and the mode of construction of these Phxo- 
daria is like that of the Thalamophora. Acanthin appears, with regard 
to its solidity, to hold a middle place between carbonate of lime and 
silicic acid, at least this holds with regard to the habit of the Acantharian 
skeletons, which, on the one hand, are more differentiated and elegant 
than the Thalamophoran shells, without, however, on the other, attaining 
the light construction and great complication of the siliceous skeletons of 
the Polycystina and Pheodaria. 
