96 THE MICROSCOPE AND ITS REVELATIONS. 



' marginal cord/ as shown in Fig. 337. 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 

 which 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, sometimes of rows of tubercles, which mark the posi- 

 tion of the septa beneath. These non-tubular plates or columns are 

 often traversed by branches of the canal-system, as seen at g, q. Similar 

 columns of non-tubular substance, of which the summits show them- 

 selves 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 characterize distinct species, were it not that on a com- 

 parison 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. 



FIG. 333. 



A, piece of Nummulitic Limestone from Pyrenees, showing Nummulites laid open by fracture 

 through median plane; B, vertical section of Nummulite; c. Orbitoides. 



489. The Genus Nummulina, of which the fossil forms are commonly 

 known as Nummulites, though represented at the present time by small 

 and comparatively infrequent examples, was formerly developed to a vast 

 extent; the Nummulitic Limestone chiefly made-up by the aggregation 

 of its remains (the material of which the Pryamids are built) forming a 

 band, often 1,800 miles in breadth and frequently of enormous thick- 

 ness, 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, 333). The diameter of a large pro- 

 portion of fossil Nummulites ranges between half an inch and an inch; 

 but there are some whose diameter does not exceed l-16th 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 flat- 

 tened; 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 Operculincz in their mode 

 of growth, yet the typical forms of this genus present certain well-marked 

 distinctive peculiarities. Each convolution is so completely 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 



