314 Mr. J. J. Lister. On the Dimorphism of — [Mar. 2, 
megalospheric form, when full grown, attains a size (test included) over 
8000 times as large as that of the initial chamber in which it began. In 
complanatus the proportion of these volumes is only 66 to 1. : 
In the approach to equality in the sizes of the tests of the two forms the 
species V. Orbignyi and variolarius, though exceptional among nummulites, 
agree with the majority of the foraminifera in which dimorphism has been 
recognised. The genus Polystomella is an example among the Vummulitide, 
in which the two forms are also of equal size. On the other hand, in 
Heterostegina, probably in Cycloclypeus and in at any rate several of the 
species of Orbitoides,* the microspheric form preponderates over the megalo- 
spheric, as it does in most of the nummulites. Outside the Vuwmmulitide 
we meet with the preponderance of the microspheric form in certain genera 
of the Miliolide (Biloculina, Miliolina, and in Orbitolites complanata), and 
it would be interesting to learn how far a similar correspondence in size 
between the microspheric form and the megalosphere obtains in these cases, 
and whether the repetition of the megalospheric generation produces any 
modification of the results. 
These stages in the reduction of the megalospheric or gamete-producing 
generation are interesting from a wider biological standpoint as affording 
a parallel with what has occurred in other groups of animals and plants. 
Thus, to take a particular instance, we may compare the small and short- 
lived Nummulites Tchthatchefi, a dwarf beside the great disc of WV. com- 
planatus, with the prothallus of a fern, arising asexually from a spore, and 
ultimately producing a zygote (by the union of gametes) which grows into 
the long-lived and comparatively gigantic sporophyte. 
When the nuclear history of the foraminifera comes to be more perfectly 
known, it will be interesting to learn how far it runs parallel in two so 
widely separated forms. At present our knowledge of it is too incomplete to 
allow a comparison to be profitably instituted. 
While writing the first part of this paper, I was beset by a suspicion that 
I was perhaps making too much of the difficulties raised by de la Harpe, that 
everyone who is interested in the life-history of the foraminifera was con- 
vinced long ago of the general prevalence of dimorphism, and that I might 
therefore have set out to fight the already slain. 
That this is not the case is shown by a memoir which has just reached me 
* See the series of papers published by M. Schlumberger on the subgeneric groups 
Orbitoides, Orthophragmina, and Miogypsina (19) and the memoir by MM. Lemoine and 
R. Douvillé on Lepidocyclina (12). 
