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On the Dimorphism of the English Species of Nummulites, and the 
Size of the Megalosphere in relation to that of the Microspheric 
and Megalospheric Tests in this Genus. 
By J. J. Lister, M.A., F.R.S., Fellow of St. John’s College and Demonstrator 
of Comparative Anatomy in the University of Cambridge. 
(Received March 2,—Read March 16, 1905.) 
[PuatEes 3—5.] 
(1) Zhe Dimorphism of the English Species of Nummulites. 
Before the dimorphic character of many of the species belonging to the 
higher groups of the Foraminifera was established, some of the phenomena 
dependent on it had been recognised among the fossil nummulites, and 
attention was called to the peculiar mode of occurrence in pairs of what were 
then. regarded as the different species of Nummulites and Assilina, in the 
beds in which they abound. 
It was especially by the careful labours of de Hantken and de la Harpe 
that this phenomenon was brought to light, and the “ Law of the Association 
of Species in Pairs” was formulated by the latter in his ‘Etude des Nummu- 
lites de la Suisse’ (5, p. 63), as follows :—* Les nummulites apparaissent par 
couples ; chaque couple est formé de deux espéeces du méme groupe zoologique 
et de grandeur inégale, la grande est sans chambre centrale, la petite en a 
toujours une.” 
De la Harpe proceeds to point out that the small species is usually much 
more abundant than the large, often furnishing 90 per cent. or more of the 
whole number of specimens present in the deposit. 
In gathering materials for his ‘Etude, in which he proposed to deal not 
with the Swiss nummulites alone, but with the group as a whole, hoping to 
be able to introduce order into the “Babylonian confusion” in which he 
found it, de la Harpe received specimens of the English nummulites from 
Professor Rupert Jones, and in acknowledging them, wrote (October 10, 
1879) :— 
Seine (II) WN. variolaria from Stubbington, and (II) WV. variolaria 
from White-Cliff Bay, are exactly the same as the Belgian form. They belong 
to the type of the species. 
“Tt is remarkable that among your specimens there is not a single 
N. Heberti, which species is nearly always the companion of JV. vartoiaria. 
In Belgium and France, in the upper part of the variolaria-heds, there is 
usually 1 per cent. of NV. Heberti. In the lower beds (base of the 
