312 M. F. Dreyer on the 
majority of the single-shelled forms are perforate, and the 
ylomatic-monaxonic forms are in the minority, although they 
do not fall so far behind the others as does the perforate form- 
type among the monothalamous Thalamophora. This dis- 
tinction in the distribution of the two growth-types becomes 
still more strongly marked, however, in the many-shelled 
forms with secondary growth. Thus in the Radiolaria both 
growth-types occur widely distributed side by side, but still 
in such a manner that a preponderance of the concentric 
growth is unmistakable, while, on the contrary, in the 
Thalamophora the terminal growth-type is exclusively * 
represented. 
The cause of this different behaviour of the Thalamophora 
and Radiolaria is to be found in the fact that the two modes 
of construction in question make different demands upon the 
solidity of the material. The perforate-concentric shell-con- 
struction requires much finer material than the pylomatic- 
terminal, and therefore it happens that, while in the siliceous 
skeletons of the Radiolaria both shell-constructions are 
represented in the highest completeness and complication, 
the Thalamophora are under the necessity of producing ex- 
clusively pylomatic-terminal shells, for with their material of 
construction, which is softer in comparison with silica, it 
would not be possible for them without impairing the solidity 
of their shells to form concentric and airy skeletons like those 
of the Radiolaria ; they must make their shells thicker and 
more massive in order to give them the necessary solidity. 
It is in the essence of the perforate-concentric mode of con- 
struction that it requires to be carried out more lightly. As 
there is no principal orifice, the passage of the sarcode to the 
outer world, and in many shell-forms also between the dif- 
ferent interspaces of the shell, is consigned exclusively to 
the pores of the shell, which for the purpose of ready 
communication must not be too narrow nor the intervening 
skeletal parts too massive ; further, the union of the latticed 
spheres concentrically nested one within the other is only 
possible by means of free radial rods, which, again, must not 
exceed a certain thickness. The conditions of the pylomatic- 
terminal mode of construction are very different. Here the 
* Only one remarkable exception to this rule is furnished by Thuram- 
mina papillata, Brady, the agglutinated shell of which is composed of two 
concentric spherical shells united to each other by some radial beams 
(Brady, ‘ Challenger’ Report, pl. xxxvi. fig. 12). The stout and rather 
irregular character of this form shows us, however, that we have here to 
do as it were only with an unsuccessful attempt to imitate the light 
construction of the siliceous skeleton with a less solid material. 
