308 Mr. J. J. Lister. On the Dimorphism of [| Mar. 2, 
frequency in the beds because, in this case, the largest specimens are 
microspheric, and some of these were selected for preparing sections. 
In the megalospheric examples (Plate 5, a and a’) the value of M varies 
from 77 to 157 pw, the average being about 106 or 10 uw greater than in the 
Alum Bay specimens. 
The long diameter of the largest megalospheric specimen I have ground 
down is 2°1 mm. 
In the three microspherte specimens in which the central chamber showed 
clearly enough to be measured, the value of m is 16, 19 and 19 yw (Plate 5, 8’). 
The largest microspheric test measures 3 by 2-4 mm. There is thus in this 
species, though it does not happen to be shown in my Alum Bay specimens, 
the same tendency as was seen in JV. variolarwus for the microspheric form 
slightly to exceed the megalospheric in size. 
It is thus evident that the English species of nummulites far from presenting 
exceptions to the law of distribution in pairs, are in exact agreement with 
that law, although the difference in the sizes of the tests between the two 
forms present in a stratum is, in the cases of WV. variolarius and Orbignyt, 
little marked. 
We have seen that de la Harpe himself withdrew one of his exceptions, and 
that two others appear to have been quoted on quite insufficient grounds; and 
on looking closely into his attitude towards the theory of dimorphism, it 
appears that his failure to admit it, and insistence on cases to which, as he 
supposed, it would not apply, may be attributed to the hostility which he 
rightly felt to an adventitious hypothesis with which that theory in its 
inception was for a time laden. M. Munier-Chalmas and M. Schlumberger, 
his colleague, recognised, at a later date, that this hypothesis was untenable, 
and withdrew it (17). Had he lived to reconsider the position calmly, we 
can hardly doubt that de la Harpe would have frankly recognised the light 
which the theory of dimorphism had shed on the matter. 
(2) On the Size of the Megalosphere in Relation to that of the Microspherre and 
Megalospherie tests. 
From the measurements given above, and also from inspection of Plates 3-5, 
it is clear that there isa great difference between the size of the megalosphere 
in NV. laevigatus and its size in the two other species, and that these also differ 
in this respect from one another. From a consideration of this contrast the 
question arose: Is there a definite relation in size between the megalosphere, 
the initial chamber of the megalospheric form, and the complete test of the 
microspheric form ? 
