'1 Mr Lister, Dimorphism of the English species of Nummulites. 



initial chamber (the microsphere) 17 — 19 fx in diameter, while 

 sections of the smaller form show that they sprang from an initial 

 chamber nearly "5 mm. in diameter. 



In examples of the two other species from the beds mentioned 

 no such difference in the size of the test is displayed ; but when 

 examined in section the individuals of the species are found to be, 

 like most other Foraminifera, dimorphic in respect of the size of the 

 initial chamber. 



Among 134 specimens of N. variolaria thus examined three 

 were found to be microspheric, the diameter of the microsphere 

 being 16 — 19 fi. In the remainder (megalospheric) the diameter 

 of the megalosphere varied from 38 — 96 fi, the average diameter 

 being about 68 li. 



In N. elegans among 94 specimens two were found to be 

 microspheric, the diameter of the microsphere being 17 — 20 /*. 

 In the others the megalosphere varied from 6Q to 148 fx, the 

 average size being about 95 li. 



It is therefore clear that these species, far from invalidating 

 the conclusion that the species of Nummulites are dimorphic, are 

 in complete accord with it. We have however to recognize that 

 while in many species of the genus the microspheric form attains 

 a much larger size than the megalospheric, in others the two 

 forms attain the same size — a condition which is indeed the rule in 

 the great majority of the Foraminifera. 



