118 PROTOZOA. 



but one large aperture. 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 Eotalines and the Nummulinida, as exhibited by 

 forms like Rot alia itself, and Calcarina on the one hand, 

 and by Polystomella and AmpJiistegina on the other hand. 

 In Polystomella (fig. 18, s) the shell is lenticular, discoidal, 

 composed of successive chambers, which are prolonged into 

 wing-like ("alar") prolongations, which extend 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 to- 

 gether under the name of Nonionina ; 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. 



AmpJiistegina still more closely approaches the Eotalines, 

 with which it has sometimes been grouped. Its shell is 

 spiral and discoidal (fig. 18, t\ usually more or less inequi- 

 lateral, 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 penetrated by numerous 

 close-set, parallel, extremely minute tubules, but the " canal- 

 system " is only imperfectly developed. Brady has shown 

 that the genus occurs in the Carboniferous ; but with this 

 exception it is Tertiary and Eecent. 



Another very ancient, and more anomalous, type of the 

 Nummuline group is the Archcediscus of Mr Brady (fig. 18, r), 

 which occurs also in the Carboniferous Limestone. In this 

 curious form the test is " convoluted, rounded, more or less 

 unsymmetrical ; formed of a non-septate tube coiled upon 

 itself in a constantly varying direction ; the shell- wall trans- 

 versed by very numerous parallel minute tubuli " (Brady). 



In the genus Nummulina itself (fig. 23) the shell is coin- 

 shaped, of large size, sometimes as big as a florin, or larger, 

 composed of numerous chambers arranged on one plane in a 



