324 PALAEONTOLOGY 



5. Nummulinidae, spiral or cyclical, with subdivided 

 chambers, and great development of supplemental 

 skeleton, surface generally smooth, never spinose. This 

 family includes most of the giant Foraminifera, and most 

 of those useful as time-indices. Genera : Fusulina (Carb.- 

 Perm.), spindle-shaped, spirally coiled about the long 

 axis ; Polystomella (Cret.-Rec.), lenticular, last whorl only 

 visible externally, its chambers and their subdivisions 

 recognizable on the surface ; Nummulites or Nummulina 

 (Carb. ? Eoc.-Rec.), discoidal or lenticular (Fig. 97) ; 



d 



FIG. 97. NUMMULITES. 



rt, A^. vicaryi d'Archiac and Haime. Edge view. (Xj.) b, N. com- 

 planatus Lamarck. Edge view, (xj.) c, rf, A 7 , garansensis Joly et 

 Leymerie. c, Part of horizontal section. (X8.) d, Part of vertical 

 section. ( X 12.) (All after d'Archiac and Haime.) 



Assilina(~Eoc.), similar, but the later whorls only partially 

 overlap the earlier. These and other genera are spiral, 

 but in one sub-family, generally known as the Orbitoides, 

 the spiral mode of growth is abandoned (except in the 

 early stages of ontogeny in microspheric individuals) for 

 a cyclical growth, chambers being added in concentric 

 rings, which more or less overlap the sides : a medium 

 horizontal section shows the structure in its simplest 

 form, and enables the genera to be distinguished easily. 

 Thus the " equatorial " chamberlets of Orbitoides (Cret., 

 Upper Campanian) are rounded ; those of Orthophragmina 



