134 PROTOZOA. 



most complex and the most highly organised of all the Foraminifera. 

 In the forms included under this head, the shell is compound ; the 

 successive chambers are enclosed each in its proper wall (as diagram- 

 matically shown in fig. 26, a); there is generally a well-developed 

 " intermediate "or " supplemental " skeleton, which renders the shell 

 strong and 1 compact, and which is perforated by a " canal-system," 

 originating in the spaces between the two lamellae composing each 

 septum ; while the shell-substance is pierced by close-set and ex- 

 tremely fine tubules (fig. 37), the septa alone wanting these, so 



Fig- 37- — Transverse section of the test of Nummulina pristina, from the Carboniferous Lime- 

 stone, enlarged ioo diameters, showing the primordial chamber and the tubulated shell-wall. 

 (After H. B. Brady.) 



that contiguous chambers usually communicate by but one large 

 aperture. The lower forms of the family have a thickened and 

 finely tubulated shell-wall, but have no intermediate skeleton. The 

 form of the shell is typically a discoidal spiral or a cycloidal disc. 



There is a relationship of a decided character between the higher 

 Rotalines and the Nummufoiidce, as exhibited by forms like Rotalia 

 itself, and Calcarina on the one hand, and by Polystomella and Am- 

 phistegina on the other hand. In Polystomella (fig. 29, s) the shell 

 is lenticular and discoidal, and is composed of successive chambers, 

 which are prolonged into wing-like (" alar ") prolongations, which ex- 

 tend inwards to the centre, thus concealing the earlier turns of the 

 spire from view, while the centre itself is occupied by a solid cal- 

 careous boss, penetrated by irregular canals. The " canal-system " 

 is extraordinarily developed and very complex. Some of the simpler 

 types of Polystomella are grouped together under the name of Non- 

 ionina ; and the genus seems to make its first appearance in the 

 Upper Chalk, being well represented in the Tertiaries, and surviving 

 to the present day. 



Amphistegina still more closely approaches the Rotalines, with 

 which it has sometimes been grouped. Its shell is spiral and dis- 

 coidal (fig. 29, t), usually more or less inequilateral, each chamber 

 being saddle-shaped, and sending forth " alar " prolongations which 

 reach nearly to the centre, where is placed a solid boss. The shell- 

 substance, with exception of the septa and the central boss, is pene- 



