THE FAMILY NUMMULINID^ 239 



Genus Ampliistegina, D'Oebigny. 



Test spiral, lenticular, inequilateral ; chambers 

 equitant, the alar prolongations on one side simple, 

 on the other divided by deep constrictions, so as to 

 form supplementary lobes. Shell-wall thickened 

 near the umbilicus, finely perforated ; the exogenous 

 thickening non-tubulous, and in transverse section 

 appearing as two cones of different sizes with the 

 apices opposed. A finely ramifying canal system 

 present both in the septa and in the shell-wall. 

 Cavhoiuferoiis and Tertiary to Recent. 



Example. — A. Lessonii, D'Orbigny, 'Ann. Sci. 

 Nat.' vol. vii. 1826, p. 304, No. 3, pi. xvii. figs. 1-4. 



This is a very variable form, and the names 

 under which it has been described are nearly as 

 numerous as its variations. A. Lessonii has a test 

 which ranges in form from a thin complanate disc to 

 a subspherical or dome-shaped test. Even in one 

 dredging many gradations in form may be found. 



It is usually found in the tropical areas of the 

 Atlantic, Pacific, and Indian Oceans ; and is com- 

 monest in water of less depth than 30 fathoms, but 

 also occurs at greater depths, even down to 1,750 

 fathoms. Middle Eocene to Beccnt. (Plate 13, 

 figs. G, (J, cf) 



Genus Operculina^ D'Oebigny. 



Shell typically complanate and piano-spiral, with 

 the whole of the convolutions visible ; the earlier 

 whorls more or less embracing. Interseptal and 



