510 FORAMINIFERA I — NUMMULITES. 



of vitreous non-tubular substance, but that which lies over them, 

 continuing them to the surface of the shell, has the same cha- 

 racter; showing itself externally in the form sometimes of con- 

 tinuous ridges, sometimes 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, g. Similar columns of non-tubular substance, of which 

 the summits 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 charac- 

 terize distinct Species, were it not that, on a comparison of a great 

 number of specimens, these variations are found to be so grada- 

 tional, that no distinct line of demarcation can be drawn between 

 the individuals which present them. 



392. The Grenus Niimmulina, of which the fossil forms are 

 commonly known as Nummulites, though represented at the present 

 time by small and comparatively infrequent examples, was for- 

 merly developed 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 1800 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 over vast areas of 

 North America likewise. 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 1-1 6th 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 Operculince in their mode of growth, yet the typical 

 forms of this Genus present certain well-marked distinctive pecu- 

 liarities. 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 tha.t of the con- 

 volution it encloses 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. Nummulites 

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



