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1905. | the English Species of Nummulites, ete. 303 
Nummulites laevigatus (Brug.) (Lamarcki d’Arch.).* 
This species abounds in some of the upper Bracklesham Beds, near Selsey. 
It is an admirably clear example of the law of association of species in pairs 
or as we now say of a dimorphic species, the difference between the two 
associated forms being obvious to superficial inspection in the sizes they attain 
(Plate 3,a and 6). As the specimens lie in the beds, and still more clearly on 
separating them, by a sieve, from the scarcely hardened glauconitic sand in 
which they are contained, the full grown microspheric forms (NV. laevigatus 
(Brug.) proper) are at once distinguished by their large size. - 
The average diameter of 12 such specimens is 15°8 mm., that of 19 approxi- 
mately full-erown specimens of the megalospheric form (the WV. Lamarcki of 
d’Archiac and Haime) being 4:5 mm. 
The relative proportion of large to small forms in four samples (containing 
in all some 8000 specimens) is 1: 8:2, 1:16°7, 1:5°8, and 1:10, giving an 
average of 1:10. It must, however, be borne in mind that a certain propor- 
tion of the small specimens are young microspheric individuals. 
On grinding down examples of the microspheric form to obtain sections 
showing the initial chamber, the diameter of this is found to be in two cases 
16 and 19 uw (Plate 3, b”). 
The megalosphere is very large in this as compared with that found in the 
other English species. Among 20 specimens the mean between the long and 
short diameters varies from 355 to 695 mw, and the average is 443 yp, nearly 
4 mm. (Plate 3, a”). 
After this experience at Selsey, it was striking to find the.specimens of the 
little WV. variolarius, which are thickly scattered through a sandy band in the 
Bracklesham Beds at White Cliff Bay, and of WV. Orbignyi (wemmelensis) var. 
elegans from the base of the Barton Beds at Alum Bay in the Isle of Wight 
and at other localities, all attaining an approximately uniform size. In 
external appearance, therefore, these species showed no indication that they 
conform to the law of dimorphism. 
* In view of the fact that each of the two forms of the species of nummulites has 
been separately named the question arises (cf. M. E. van den Broeck [2]) which of the 
two names so applied shall stand as that of the species. There seems no reason for 
departing from the ordinary rules of nomenclature in this instance. I have, therefore, 
in each case selected for that of the species the name first given, whether to the 
megalospheric or the microspheric form of it. M. van den Broeck would prefer to select 
for the specific name that of the megalospheric form (which he, rather oddly, regards as 
the more “normal” form of the species) ; concluding that, this form being the more 
abundant, it will usually turn out to be that first given. As a fact, however, among the 
9 species dealt with in this paper the megalospheric form was, I believe, named before 
the microspheric in only one, viz., V. variolarius. 
