310 Mr. J. J. Lister. On the Dimorphism of — [Mar. 2, 
Nummulites complanatus, Lamk., from Hungary and Bavaria, and its 
megalospheric form, V. Tchthatchefi, d’ Arch. ; from Ajka, Vesprimer Comitat, 
Hungary. 
N. perforatus (de Monttft.), Vesprimer Comitat, Hungary, and its megalo- 
spheric form, WV. Lucasanus, Defr.; from Zirez, Vesprimer Comitat. 
NN. gizehensis (Forsk.) and its megalospheric form, N. curvispirus (Menegh) ; 
bottom of Mokattam Hill, Cairo. 
N. laevigatus (Brug.) and its megalospheric form, N. Lamarcki, d’ Arch. and 
Haime; from the Bracklesham Beds, Selsey. 
N. biarritzensis, d’Arch., and its megalospheric form, N. Guettardi, d Arch. 
and Haime; from Dahr el Nakhl, Egypt. 
NV. discorbinus (Schlot.) and its megalospheric form, WV. swb-discorbinus, de la 
Harpe; from the top of the Mokattam Hill, Cairo. 
N. Ortignyi (Gal.), var. elegans, Sow., and its megalospheric form, J. 
wemmelensis, de la Harpe and Van den Broeck, var. elegans, Sow.; from 
Barton Beds, Huntingbridge, near Fritham, Hants. 
NV. variolarius (Lamk.) and its microspheric form, WV. Hebertz, d’Arch. and 
Haime ; from the Bracklesham Beds, White Cliff Bay, Isle of Wight. 
Assilina (Nunmmulites) exponens (Sow.) and its megalospheric form 
A. mamillata (d’ Arch.) ; from Traunstein, Bavaria, 
The results of the measurements are embodied in the following table. 
The species are arranged in the order of the volumes of their megalo- 
spheres. In each case, after the index of the volume of the test, the ratio 
which this bears to the index of the volume of the megalosphere is given. 
It will be seen from the last column but one that the order of the volume 
of the megalospheres is, with one exception, the order of the volume of the 
corresponding microspheric forms. 
Turning to the results entered in the last column, the ratios of the 
volumes of the microspheric tests to those of the corresponding megalo- 
spheres, it will be seen that although the latter range from 0-0003 to 
1:161 cub. mm., the ratio is fairly constant in six cases, lying. between 9000 
and 12,500 to 1. Bearing in mind that it is the ratio of the volume of the 
protoplasmic contents of the tests to that of the megalosphere which we 
are endeavouring to estimate, the low figure representing NV. Orbignyi, var. 
elegans, may not be out of harmony with these results when the characters 
of the species are taken into account. (Compare the large chambers and 
thin walls of this species as represented in Plate 5 with those of the species 
shown in Plates 3 and 4.) 
Similarly the high figures for WV. biarriizensis and N. discorbinus are 
swollen owing to the thickness of the shell substance which builds up their « 
