Structure of Rhizopod Shells. 305 
polythalamous Thalamophora, some pylomatic Spumellaria*, 
and the Pheodarian families Challengerida, Medusettida, and 
Tuscarorida. 
Close to the eudipleural forms come the spirally-wound 
Rhizopod shells, which are to be regarded essentially only as 
a continuation of the eudipleural ground-form by the process 
of terminal growth, which will presently be referred to more 
particularly. There are therefore, especially in freshwater 
Rhizopoda, very gradual transitions from simply eudipleural 
to spirally twisted shells. In this respect the Diflugie are 
particularly instructive, as in them all transitions trom mon- 
axonic to eudipleural and from these to spiral shells are 
represented ; thus, for example, Difflugia corona is typically 
monaxonic, D. marsupiformis, with the pylom displaced 
forward, eudipleural, while, finally, D. spiralis already shows 
distinctly the half-turn of a spiral f. In the same way as in 
these first and perhaps still individually varying commence- 
ments in the freshwater Rhizopoda, the highly developed 
marine Thalamophora, often showing many spiral windings, 
have been developed, as is indicated, among other things, by 
the monaxonic central first chamber (the so-called embryonal 
chamber). ; 
Having in the preceding submitted the Rhizopod shell to a 
short consideration with regard to its form, we may now pro- 
ceed to examine it somewhat more closely from another point of 
view, namely as to the mode of its growth. In this, at the 
first glance, we meet with an interesting parallelism with the 
two torm-types just referred to. Just as in the case of these 
form-types we can also distinguish in the mode of growth of 
the Rhizopod shell two principal types, which may be placed 
side by side with the two form-types, and on the whole are 
to be conceived as a continuation of the latter caused by 
growth. Thus the perforate form-type corresponds to the 
concentric type of growth, and the pylomatic form-type to the 
terminal type of growth. 
The concentric growth-type, as implied by its name, con- 
sists in that the soft body during its further growth around 
its first spherical perforate shell, which gradually becomes 
too small for it, separates externally successive larger con- 
centric spherical shells. The shells of such a system of 
latticed spheres nested one within the other are bound together 
* The bilaterality indicated in a great number of Nassellaria by the 
relations of the basal and apical spines is original and does not belong to 
this category. See ‘ Radiolarienstudien,’ Heft i. p. 100, note 2. 
t See ‘ Radiolarienstudien,’ Heft i. Taf. vi. figs. 88, 89, 90. 
Ann. & Mag. N. Hist. Ser. 6. Vol. iv. 21 
