100 PKOTOZOA. 



the shell is " porcellanous," homogeneous, and opaque-white 

 when viewed by reflected light. On the other hand, in those 

 of the calcareous Foraminifera in which the walls of the 

 test are perforated by pseudopodial apertures, the shell is 

 " vitreous " or " hyaline," transparent and glassy, and often 

 of a thin and delicate texture. The " arenaceous " Foramin- 

 ifera are normally and typically " imperforate ; " but Mi- 

 Henry Brady has shown that there exist forms (such as 

 Valvulina, Nodosinella, and Endothyrci) in which the texture 

 of the shell is arenaceous or sub-arenaceous, but the walls 

 of which are sometimes porous, though more usually imper- 

 forate. We are thus presented with a series of intermediate 

 forms by which the gap between the Perforata and the Im- 

 perforata is to some extent bridged over. 



Thirdly, as regards the form of the shell, great differences 

 exist among the Foraminifera, and as concerns the mere 

 external configuration, this is so variable that little or no 

 value can be attached to it in classification. Moreover, in 

 the two great series of the Perforate and the Imperforate 

 Foraminifera it is common to find parallel or u isomorphic " 

 groups. That is to say, we meet with two series of forms, 

 repeating each other's peculiarities and variations in form, 

 but the shell in the one series being perforate, while in the 

 other it is imperforate. 



The simplest form among the Foraminifera is that of a 

 single spheroid of sarcode, capable of secreting for itself a 

 hard covering, as in the flask-shaped Lagena 

 (fig. 13, ) or the globular Orbulina (fig. 14). 

 Forms such as these are said to be " uni- 

 locular" or " monothalamous," the test con- 

 sisting .of but a single chamber, not sub- 

 divided by partitions or " septa." In the 



Fig. 14. Or&wMua J , r . . . \ 



A simple more complex foramimfera, .the sarcode 01 

 the bod y undergoes a subdivision into par- 

 beds) of tially separated segments, which may be pro- 



Italy. (D'Orbigny.) , V, ,* * i -, i , 



duced by a process of budding, or, perhaps, 

 by the occurrence of constrictions in the growing protoplasm, 

 and each of these segments becomes more or less com- 

 pletely divided off from its neighbours, or enclosed by a 



