SECT. ir. THE NUMMULITES. 45 



of its canal system. Their forms vary from that 

 resembling a nautilus or ammonite to a flat spiral or 

 cyclical disk, like an Orbitolite, though vastly superior 

 to it in organization both with regard to the animal 

 and to the structure of the shell. 



All the species of the genus Nummulite are spiral ; in 

 the typical form the last turn of the spire not only 

 completely embraces, but entirely conceals, all that 

 precede it. In general, the form is that of a double 

 convex lens of more or less thickness ; some are flat, 

 lenticular, and thinned away to an acute edge, while 



Fig. 101. Section of Fanjasina. 



others may be spheroidal with a round, or obtuse edge. 

 They owe their name to their resemblance to coins, 

 being, in general, nearly circular. Their diameters 

 range from T ybh of an inch to 4 inches, so that they 

 are the giants of their race ; but the most common 

 species vary from ^ an inch to 1 inch in diameter. 



Fig. 101 represents a section of the Nummulite 

 Faujasina near and parallel to the base of the shell. 

 It shows a series of chambers arranged in a flat 



