FORAMINIFERA. 



119 



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 successive chambers com- 

 municate by means of arched fissures, which perforate each 

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



Fig. IZ.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. 



spire, while secondary and irregular pores in the septa dis- 

 charge the same function. The general shell-substance is 

 traversed by extremely minute parallel tubuli (fig. 23, D) ; 

 and there is a supplemental skeleton (forming the so-called 

 " marginal cord "), which, together with the septa, is pene- 

 trated by a well-developed and ramified " canal-system " 

 (see fig. 17, D). By the researches of Mr Henry Brady, we 

 know now that the range of the genus Nummulina in time 

 must be carried back to the Carboniferous, one small form 

 having been detected in the Mountain Limestone of Belgium. 

 A few " Nummulites " have also been detected in strata of 

 Jurassic and Cretaceous age, but the maximum development 

 .of the genus is recorded in the early Tertiary period (Middle 



