NUMMULITES 831 



cation between the chambers ; in most of the septa, however, there 

 are also some isolated pores (to which the lines point that radiate 

 from e, e) varying both in number and position. The inter septal 

 canals of each septum take their departure at its inner extremity 

 from a pair of spiral canals, of which one passes along each side of 

 the marginal cord ; and they communicate at their outer extremity 

 with the canal system of the ' marginal cord,' as shown in fig. 634. 

 The external walls of the chambers are composed of the same finely 

 tubular shell-substance that forms them in the Nummulite ; but, as 

 in that genus, not only are the Septa themselves composed of vitreous 

 non-tubular substance, but that w r hich lies over them, continuing 

 them to the surface of the shell, has the same character, showing 

 itself externally in the form sometimes of continuous ridges, some- 

 times of rows of tubercles, which mark the position of the septa 

 beneath. These non-tubular plates or columns are often traversed 

 by branches of the canal system, as seen at </, g. Similar columns 

 of non-tubular substance, of which the ."ummits show themselves as 

 tubercles on the surface, are not unfrequently seen between the 

 septal bands, giving a variation to the surface-marking which, taken 

 in conjunction with variations in general conformation, might be 

 fairly held sufficient to characterise distinct species, were it not that 

 on a comparison of a great number of specimens these variations 

 are found to be so gradational that no distinct line of demarcation 

 can be drawn between the individuals which present them. 



The genus Nummulites, though represented at the present time 

 by small and comparatively infrequent examples, was formerly de- 

 veloped to a vast extent, the Nummulitic limestone, chiefly made up 

 by the aggregation of its remains (the material of which the Pyramids 

 are built), forming a band, often 1,800 miles in breadth and frequently 

 of enormous thickness, that may be traced from the Atlantic shores 

 of Europe and Africa, through Western Asia to Northern India and 

 China, and likewise over vast areas of North America (fig. 630). 

 The diameter of a large proportion of fossil Nummulites ranges 

 between half an inch and an inch ; but there are some whose 

 diameter does not exceed T ^th of an inch, whilst others attain the 

 gigantic diameter of 4^ inches. Their typical form is that of a 

 double-convex lens ; but sometimes it much more nearly approaches 

 the globular shape, whilst in other cases it is very much flattened ; 

 and great differences exist in this respect among individuals of what 

 must be accounted one and the same species. Although there are 

 some Nummulites which closely approximate Opereulince in their 

 mode of growth, yet the typical forms of this genus present certain 

 well-marked distinctive peculiarities. Each convolution is so com- 

 pletely invested by that which succeeds it. and the external wall or 

 spiral lamina of the new convolution is so completely separated from 

 that of the convolution it 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 Num- 

 mulite through the median plane, which may often be accom- 

 plished simply by striking it on one edge with a hammer, the opposite 



