PROCEEDINGS 



OF THE 



C&mMtrg* f^jwtopfjral Soring, 



On the Dimorphism of the English species of Nummulites. 

 By Mr J. J. Lister. 



[Bead 31 October 1904.] 



The author gave an account of his examination of the characters 

 of three English species of Nummulites, N. laevigata (Brug.), vario- 

 laria Lam., and elegans Sow., with respect to dimorphism. 



De Hantken and de la Harpe called attention to the general 

 occurrence of Nummulites in pairs in the same strata, and the 

 phenomenon was referred to by them as the ' association of 

 species/ 



When in 1880 Munier-Chalmas propounded his view of the 

 dimorphism of the species of Nummulites and of other Foramin- 

 ifera, it was objected by de le Harpe that though association in 

 pairs was the rule, there were several instances of the occurrence 

 of a single form ("privee d'une sceur") in a deposit ; a circum- 

 stance which, if established, would be difficult to reconcile with 

 the theory of dimorphism. Although that theory is now firmly 

 established the difficulty raised by de la Harpe appears never to 

 have been met. N. variolaria, in the Bracklesham Beds of White 

 Cliff Bay, and N. elegans, from the Barton Beds in Alum Bay, 

 have been described, as they occur in these beds, as such solitary 

 forms, although in deposits on the Continent they have both been 

 found associated with a, companion ' species.' It therefore seemed 

 worth while to examine them to find what support they might 

 have to give to de la Harpe's objection. 



In N. laevigata, the two forms of the species are readily dis- 

 tinguished when fully grown, owing to the fact that the micro- 

 spheric form attains a much larger size than the megalospheric. 

 A section in the median plane of such large specimens reveals an 



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