FOKAMINIFERA AND RADIOLAKIA. 97 



incloses by the ' alar prolongations ' of its own chambers (the peculiar 

 arrangement of which will be presently described), that the spire is 

 scarcely if at all visible on the external surface. It is brought into view, 

 however, by splitting the Nummulite through the median plane, which 

 may often be accomplished simply by striking it on one edge with a 

 hammer, the. opposite edge being placed on a firm support; or, if this 

 method should not succeed, by heating it in the flame of a spirit-lamp, 

 and then throwing it into cold water or striking it edgeways. Nummu- 

 lites usually show many more turns, and a more gradual rate of increase 

 in the breadth of the spire, than Foraminifera generally; this will be 

 apparent from an examination of the vertical section shown in Fig. 334, 

 which is taken from one of the commonest and most characteristic fossil 

 examples of the genus, and which shows no fewer than ten convolutions 

 in a fragment that does not nearly extend to the centre of the spire. 

 This section also shows the complete inclosure of the older convolutions 

 by the newer, and the interposition of the alar prolongations of the 

 chambers between the suc- 

 cessive layers of the spiral FlQ - 334< 

 lamina. These prolonga- 

 tions are variously arranged 

 in different examples of the 

 genus: thus in some, as N. 

 distant, they keep their own , 

 separate course, all tending' 1 

 radially towards the centre; 

 in others, as N. Icevigata, 

 their partitions inosculate 

 with each other, so as to di- _^^^ 



Vide the Space intervening Vertical section of portion of Nummulina Icevigata;- 

 between each lavei* and the ' mar ^ in of external whorl; &, one of the outer row of 

 . J i chambers; c, c, whorl invested by a; d. one of the chambers 



next into an UTegUlar n et- of the fourth whorl from the margin; e, e'. marginal por- 

 wnrlc nrp<5PnHno- in vprfioal tions of tne inclosed whorls; /, investing portions of outer 

 W01K, pit ing 11 Vemcai whorl . ^ ^ spaces left between the investing portion of 



Section the appearance.showil successive whorls; h, h, sections of the partitions dividing 



in Fig. 334; whilst in N. gar- these " 



ansensis they are broken up into a number of chamberlets, having little 



or no direct communication with each other. 



490. Notwithstanding thab the inner chambers are thus so deeply 

 buried in the mass of investing whorls, yet there is evidence that the 

 segments of sarcode which they contained were not cut off from commu- 

 nication with the exterior, but that they may have retained their vitality 

 to the last. The shell itself is almost everywhere minutely porous, 

 being penetrated by parallel tubuli which pass directly from one surface 

 to the other. These tubes are shown, as divided lengthways by a ver- 

 tical section, in Fig. 335, a, a ; whilst the appearance they present when 

 cut across in a horizontal section is shown in Fig. 336, the transparent 

 shell-substance a, a, , being closely dotted with minute punctations 

 which mark their orifices. In that portion of the shell, however, which 

 forms the margin of each whorl (Fig. 335, b, V), the tubes are larger, 

 and diverge from each other at greater intervals; and it is shown by 

 horizontal sections that they communicate freely with each other later- 

 ally, so as to form a network such as is seen at b, b, Fig. 337. At cer- 

 tain other points, d, d, d (Fig. 335), the shell-substance is not perfo- 

 rated by tubes, but is peculiarly dense in its texture, forming solid pil- 

 lars, which seem to strengthen the other parts; and in Nummulites whose 

 7 



