316 Mr. J. J. Lister. On the Dimorphism of — [Mar. 2, 
unnecessary to introduce so complex and incomprehensible a conclusion into 
our ideas on the life-history. 
That the megalospheric form in some: families may reproduce its like for 
one, and very possibly for a series of generations, is well known, but where 
is the evidence that the microspheric form ever reproduces the microspheric 
directly ? I have never seen or read of one particle of evidence to this effect. 
If dimorphism is due “in certain cases” to alternation of generations, to 
what other mode of life-history or cause of any kind is it to be attributed in 
closely allied forms ? 
A little further on the authors cite a paper (8A) by M. E. Haug, in which 
he simply records the fact that, among a number of examples of WV. variolarws 
in a certain bed, he did not find the microspheric form (WV. Heberti); though 
he is far from drawing the conclusion, which the authors are inclined to 
draw, that it was, in fact, absent. De la Harpe found, as we have seen, that 
it is impossible to distinguish this form from the megalospheric, by examina- 
tiun of whole specimens, and in my own experience it was not till I had ground 
down 45 specimens of this species, all of which proved to be megalospheric, 
that I happened on an example of the microspheric form. I cannot admit, 
therefore, that such negative evidence, unless supported by observations on 
the size of the initial chambers of a large number of specimens, has any force 
to shake a conclusion so firmly established. 
If I put these objections abruptly, I beg the authors to believe that I do so 
in no hostile spirit, but for the furtherance of the subject in which we are 
alike interested. It seems to me that we are in danger of letting drop the 
clue which is within our grasp. 
Summary. 
The results obtained in this investigation may be summarised as 
follows :— 
1. Both microspheric and megalospheric forms of NV. variolarius and 
N. Orbignyi var. elegans are present in the Eocene Beds of the Isle of Wight 
and Hampshire, as I believe they will be found to be present elsewhere, 
except when the materials of a bed have been rearranged under the influence 
of currents. 
2. In these species and in UW. laevigatus and N. gizehensis the size of the 
microsphere is nearly constant—the diameters in the specimens measured 
being between 15 and 20 yp. 
3. In the nine species and one variety of Nummultes and Assilina which I 
have examined, the size of the megalosphere is approximately proportional 
to the volume of the contents of the microspheric form. 
