302 M. F. Drejer 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 E.adiolaria 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. This development of the shell is extraordinarily 

 diffused in the most different divisions of the Rhizopoda, and 

 it may be regarded as characteristic of the raonaxon-pylomatic 

 type. Corresponding forms occur in Dijfiu(/ia, Euglyphaj 

 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 of 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- 



