302 M. F. Dreyer on the 
It is otherwise, however, with the pylomatic type. Hand 
in hand with the development of a chief aperture or pylom a 
series of transformations and differentiations occur in the 
Rhizopod shell, and these become particularly interesting 
because they are independent of the material of which the 
shell is composed and are developed independently in the most 
different groups of the Radiolaria and Thalamophora. From 
this it follows that here we have to do with purely analogical 
structures, which, standing in correlation with the formation 
of the pylom, occur only in the Rhizopod shells which are 
distinguished by a principal orifice. It may therefore be 
profitable to go somewhat in detail into these peculiarities of 
the monaxonic-pylomatic type. 
The most usual accompaniment of the formation of the 
pylom is an elongation of the shell in the direction of the 
principal axis—more rarely this axis is abridged. If the 
shell possesses radial skeletal elements, spines, &c., a corre- 
sponding influence makes itself felt even in these—they 
arrange themselves, following the direction of the principal 
axis, in such a manner that those of the oral half of the shell 
are directed towards its oral pole and those of the aboral half 
towards the aboral or apical pole. Generally this process of 
differentiation goes still further, inasmuch as on the equatorial 
parts of the shell no spines are developed, but they are con- 
fined to the two poles. Then is produced an elongated, 
elliptical, or oval shell, one pole of which is occupied by the 
principal aperture. Further, the two poles of the principal 
axis are distinguished by radial’ spines or other structures; at 
the oral pole these surround the pylom as radial marginal 
ornamentations of various kinds, while the opposite apical 
pole is furnished either with a tuft of spines or with some 
generally regularly grouped spines, or with a single strong 
apical spine. ‘I'his development of the shell is extraordinarily 
diffused in the most different divisions of the Rhizopoda, and 
it may be regarded as characteristic of the monaxon-pylomatic 
type. Corresponding forms occur in Deflugia, Huglypha, 
Quadrula, Campascus, Lagena, in numerous polythalamous 
Thalamophora, and most generally diffused in the Nassellaria, 
pylomatic Spumellaria, Challengerida, Circoporida, Tusca- 
rorida, Medusettida, and Castanellida. 
Instead of the marginal spinosity the pylom is sometimes 
produced into a tube. In many a pylom occurs also at the 
aboral pole, so that the shell, perforated by a mouth at both 
poles ot the principal axis, acquires an amphistomous charac- 
ter. All these morphological characters of the monaxon- 
pylomatic type are allied phenomena and stand in close corre- 
