231 



CHAPTEE XVI 



THE FAMILY NUMMULINIDiE 



In this family the foraminiferal shell exhibits its 

 highest structural development. One of the chief 

 characters of three of its sub-families is the presence 

 of a supplemental skeleton with a more or less com- 

 plex interseptal canal system. The shell-wall is 

 always finely perforated. The general plan of the 

 shell is spiral and equilateral ; some exceptions to 

 this are A'jnjyJiistegina, AixIlcecHscus, and Fatijasina : 

 or cyclical, as in Cycloclyjmus and Orhitoides. The 

 tests in the Nummulinid.e are either discoidal, lenti- 

 cular, ovate, or fusiform in contour. 



The isomorphous forms in this group which agree 

 with those of other divisions according to some general 

 plan of growth are quite numerous, and we may quote 

 for examples Fusulina(=Alveolina) and Gyclochjpeus 

 (= certain species of Orhitolites). 



From a geological point of view this family is 

 perhaps the most important of the entire order, since 

 nearly all its members are active agents in the for- 

 mation of limestone rocks of considerable extent. 

 Among the more noteworthy of these the genera 

 Fusulina, AmjyJiistegina, Operculiita, Heterosteginciy 

 Nummulites, and Orhitoides may be mentioned. 



