314 M. F. Dreyer on the 
external, continuous shell-closure is developed only after the 
completion of the growth of the spongy disk *. 
As we have seen, the agglutinated and calcareous materials 
agree in that, as compared with the silica, they possess 
less firmness, the consequence of which is that the Thalamo- 
phoran shells are more compactly and simply constructed than 
the siliceous skeletons of the Radiolaria. On closer exami- 
nation, however, a distinction may be recognized between the 
agelutinating and calcareous Thalamophora, consisting in the 
fact that the former are more coarsely and simply constructed 
than the latter, and this is certainly due to the agglutinated 
constructive material being inferior in solidity to the homo- 
geneous calcareous mass. Although this difference is not so 
great as that between Thalamophoran and Radiolarian shells, it 
nevertheless exists, and toall appearance its importance must not 
be undervalued. Quite recently Neumayr has specially called 
attention to this circumstance, and made use of it for a phylo- 
geny of the Thalamophora, assuming the more highly differ- 
entiated calcareous-shelled forms to have become developed 
from the simple arenaceous-shelled types as their stem-forms fT. 
It will be most convenient, in the first place, to reproduce this 
theory of Neumayr’s in the author’s own words. He says :— 
“The low forms furnished with the most imperfect shell- 
structure which form Brady’s very well-founded family 
Astrorhizide are exclusively sandy ; the most highly deve- 
loped Foraminifera, furnished with a branched canal-system, 
double septa, an intermediate skeleton, &c., are exclusively 
caleareous ; while the forms standing between the two are 
partly sandy, partly calcareous, and show many transitions 
from one development to the other. This condition of things 
leads to the supposition that arenaceous forms, without any 
trace of a complicated structure, such as we find in the Astrorhi- 
zidx, represent the stem-types from which the other Forami- 
nifera have been developed. ... In favour of the notion that 
the arenaceous Foraminifera in reality represent the original 
type, we have in the first place their geological occurrence, 
inasmuch as they occur in old deposits in comparatively much 
greater number than subsequently ; it is true that in the com- 
parison of the living with the Tertiary and Mesozoic species 
this does not appear so strikingly, but it is perfectly distinct 
* See for further details my ‘Pylombildungen,’ Abschnitt vy. Taf. y, 
figs. 64-69, and Taf. vi. figs. 97-100. 
t Neumayr, “ Die natiirlichen Verwandschaftsverhaltnisse der schalen- 
tragenden Foraminiferen,” in Sitzungsb. Wien. Acad. Bd. xev. Abth. 1 
(1887), and also in ‘ Die Staimme des ‘Thierreichs,’ Bd. i (1889). 
