308 M. F. Dreyer on the 
Having now briefly indicated the relation of the poly- 
thalamous to the monothalamous forms, the question naturally 
occurs to us which of the latter, the shells without secondary 
growth, are to be regarded as the most primitive. A careful 
investigation of the conditions coming under consideration 
shows us that a positive answer to this question cannot be 
given. The perforate, more or less homaxonic Monothalamia 
in almost all cases show a primitive character; but this may 
also be assumed with a very high degree of probability for 
many pylomatic Monothalamia. On the other hand, it is 
exceedingly probable that a great part of the pylomatic 
Monothalamia have only arisen secondarily from perforate 
spherical forms. This view is supported especially by some 
important transition-forms which occasionally occur. Thus 
the number of pores in the spherical shell of Mcerocometes 
varies from 5 to 1, so that in the latter case we have already 
the indication of a monaxon-pylomatic development; and in 
Thurammina and Orbulina one shell-pore is sometimes dis- 
tinguished from the rest by its greater size. In Radiolaria 
the secondary origin of a pylom occurs very widely, and 
with regard to this I may refer to the detailed treatment of 
the point in my ‘ Radiolarienstudien.’ 
Whilst, therefore, one form-type may pass over into the 
other, this is by no means the case with the. growth-types. 
It never happens that a form which has grown terminally for 
a time afterwards adopts the concentric growth, or the reverse. 
According to extant observations at least it may pass as an 
unexceptional rule that the same form always remains true to 
the growth-type which has once been adopted. ‘The beha- 
viour of the pylomatic Spumellaria is particularly instructive 
upon this pomt. Not only in many single-shelled Spumel- 
laria, but also in many in which several concentric spherical 
or annular systems are already present, a pylom is developed ; 
but nevertheless these forms continue without disturbance to 
grow concentrically, the influence of the pylom not being of 
sufficient importance to suppress the concentric growth and 
cause the shell to continue its growth terminally. ‘The 
Rhizopoda in question are able to change their form-type, 
but not their growth-type. 
In what has been said mention has several times been made 
of developmental or transformational processes in the Rhizo- 
pod skeleton ; with regard to these the following must also 
be brought to mind. For the genetic explanation of the 
innumerable phenomena of differentiation three possibilities 
have been given in accordance with the different particular 
results. A great number of structures are referable to simple 
