
On the Dimorphism of the English Species of Nummulites. 299 
‘Laekenian’) these are more numerous (up to 50 per cent.). Is it that 
Heberti is totally absent, or that the number of the specimens you kindlyjsent 
me is still too few for strict comparison? This is an interesting question to 
solve. 
“(IV) My best thanks for NM. Prestwichiana, R, Jones....,.I1 am 
now working out the Belgian Nummulites. I find in that country eight 
species, very distinct and easy to separate,” 
Here follows the list :— 
i es planulata, Lamk., “without central chamber.’ 
N, elegans, Sow., “with central chamber.” 
IL et laevigata, Lamk., “without central chamber.” 
N. Lamarcki, VArch., “with central chamber.” 
rn Heberti, VArch., “without central chamber.” 
N. variolaria, Sow., “with central chamber.” 
N. @Orbigny, Gal., “without central chamber.” 
Iw Wemmelensis (or “with central chamber.” 
Prestwichit). 
De la Harpe continues: “They make four pairs, of two species each, one 
species being without, the second with the central chamber ...... 
“ As far as I know, NV. Heberts and Orbignyi have not yet been observed in 
England. I should be very much surprised if they do not exist in the same 
beds with WV. variolaria and Wemmelensis (Prestwichi). Their absence 
would be a remarkable exception to the general law of the distribution of 
Nummulites.” The law is then stated nearly in the form given above 
[10, Supl. Note viii, 2]. 
Although de la Harpe had done so much to bring to light the essential 
facts of the case, he had suggested no explanation of this remarkable law. 
The question of its significance remained for him “sans solution.” 
In the following year (1880) Munier-Chalmas brought before the 
Geological Society of France his conclusion, based on the study of four fossil 
species of Nummulites and two of Assilina, that the species of these genera 
are dimorphic, and that the phenomenon of dimorphism would be found to be 
general (15). The explanation thus offered of the phenomenon of the 
distribution of the “species” in couples, was that the members of the couple 
were not, in fact, of distinct species, but that they represented two forms of 
a single species. , 
So far, Munier-Chalmas’ conclusion was, as subsequent investigations have 
abundantly proved, entirely right. It has been shown by observations on 
living foraminifera (13, 14, and 16) that the form with the large central 
x 2 
