316 M. F. Dreyer on the 
corresponding isomorphous calcareous forms are directly 
connected, while the most highly developed and most differen- 
tiated terminal types of the stems are exclusively of calcareous 
nature. 
This phylogeny of the Thalamophora of Neumayr’s har- 
monizes perfectly with our conception of the significance of 
the structural material of the Rhizopod shell, and the two 
theories lend each other a support which must not be under- 
valued. The lower and lowest forms find the coarse aggluti- 
nated material quite sufficient for the construction of their 
simpler shells; the forms of medium complication already 
for the most part have recourse to carbonate of lime; while, 
finally, the most differentiated types construct their shells 
exclusively of lime, because this finer and firmer material 
alone renders possible that complicated structure which could 
not be carried out with the coarse and less solid agglutinated 
material. Only in the case of one of Neumayr’s assumptions 
I should consider a certain limitation necessary. As appears 
from the last of the passages above cited, Neumayr regards 
the more imperfect and coarser construction of the arenaceous 
forms in comparison with the isomorphous calcareous ones as 
a primitive condition, and a special proof that the arenaceous 
forms are to be regarded as forerunners of the calcareous. In 
most instances, in all probability, this is the case, but not 
without exception. It is possible, nay, highly probable, that, 
as at the present day, the shell-material varies in certain forms 
with changes of the external conditions under which the 
Rhizopoda in question live; this has also occurred now and 
again during the phylogenetic development, and calcareous 
forms may thus be compelled to make their shells of sand. 
These will then, in consequence of the coarser material, 
appear ruder and less differentiated than the calcareous stem- 
form. Although the sarcode-body of such forms will have 
inherited the tendency to secrete hard parts equally well- 
developed morphologically, it will be unable, on account of 
the coarser nature of the sandy material, to bring this faculty 
to full development, as was the case with the calcareous 
material. Just as the Thalamophora in the course of their 
phylogenetic development were compelled, for the purpose of 
the higher morphological development of their shells, to pass, 
independently in the different stems, from the agglutinated 
material, which no longer sufficed for this purpose, to carbo- 
nate of lime, a form which is under the necessity of going 
back from the calcareous to the arenaceous development will 
also show a corresponding retrogression in respect of morpho- 
logy. Such a change of material, as also the existence of 
