FORAMINIFERA. 



135 



trated 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 Recent. 



Another very ancient, and more anomalous, type of the Nummu- 

 line group is the Archadiscus of Dr Brady (fig. 29, 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 transversed by very numerous parallel minute tubuli " 

 (Brady). 



In the genus Nummulina itself (fig. 38) the shell is coin-shaped, 

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



Fig. 38. — Nummulina nummularia. A, The shell viewed from above ; B, The same, 

 horizontally bisected ; c, The same vertically bisected ; d, Vertical section of part of the 

 shell, highly magnified, showing the chambers of the median plane, the alar prolongations, 

 and the tubuli of the shell-substance. Eocene Tertiary. 



numerous chambers arranged on one plane in a regular spiral. Each 

 chamber is saddle-shaped, the internal or " alar " prolongations of 

 each extending to the centre, so that each revolution completely 

 encloses and conceals from view all the preceding ones. The suc- 

 cessive chambers communicate by means of arched fissures, which 

 perforate each septum, close to the periphery of the previous turn of 

 the spire, while secondary and irregular pores in the septa discharge 

 the same function. The general shell-substance is traversed by ex- 

 tremely minute parallel tubuli (fig. 38, d, and fig. 37) ; and there is 



