Structure of Rhizopod Shells. 317 
isomorphous arenaceous and calcareous forms, occurs, how- 
ever, only in Thalamophora moderately high in development, 
and, indeed, is possible only in them because here the corre- 
sponding morphological change extends only to unimportant 
peculiarities, but impossible in the highest and most differ- 
entiated types, such as the Nummulites for example, in which 
a reversion to the arenaceous grade of development would 
need to be accompanied by a profound change in the whole 
structure of the shell. 
Thus, then, we have seen that in the three principal 
materials which come under consideration in connexion with 
the hard structures of the Rhizopoda, so many degrees of 
firmness and fineness may be recognized, which exert a very 
considerable influence upon the structure of the shells and 
skeletons. If we would illustrate these conditions by an 
example out of everyday life, we may fairly compare the 
agglutinated arenaceous material, the carbonate of lime, and 
the silicic acid, as the materials of the Rhizopod shells, on the 
one hand, with mud, stone, and iron, the three most important 
substances in the buildings made by man. The mud-struc- 
tures, like the arenaceous Rhizopod-shells, can be carried out 
only in a rough and more or less primitive manner, like the 
birds’ nests (such as those of the Swallows) built of mud, 
owing to the coarse texture and want of solidity in the 
material employed ; in fact, the mutually adherent chambers 
of many “ Agglutinantia” among the Rhizopoda possess a 
remarkable resemblance to the Swallows’ nests aggregated 
together on the wall of a house. Stone-buildings and the 
calcareous Rhizopod shells take an intermediate position ; 
while the siliceous skeletons of the Radiolaria and the infi- 
nitely varied iron structures of everyday life, from the great 
solidity of the materials, give the greatest room for compli- 
cation and differentiation, and at the same time for multipli- 
city of form. It is not only the inherited faculty of the soft 
body to construct more or less complicated and differentiated 
skeletal parts that regulates the shell-structure, but, like 
human architects, the Rhizopoda are also more or less depen- 
dent upon their material, and must deal with its peculiarities. 
As we have already seen, the concentric growth makes 
greater demands than the terminal upon the firmness of the 
material, and it is therefore met with only in the siliceous 
Radiolarian skeletons, while it does not occur among the 
Thalamophora. But at the same time the concentric skeletal 
structure has an advantage of which the terminal is destitute. 
A system of several nested spherical shells or parts of such 
shells forms an externally closed rounded whole which presents 
