THE FORAMINIFERA 



59 



veniently used for this purpose. In what follows I have used the 

 term multiform to cover any departure from the uniform condition 

 of growth. 



It is shown below that while in some genera both forms of 

 individual which are found in one species are alike bi- or tri-formed, 



FIG. 5. 



Block of Eocene limestone, showing the two forms of a species of Nummulites, constituting 

 "a pair" ; the larger named N. biarritzensis, the smaller N. guet.tfi.nli, d'Arch. Specimen from 

 Deir en Xakhl, Egypt, in the Brady Collection, Cambridge. 



in others the phenomenon is exhibited by only one form (the 

 microspheric), or exhibited by it to a greater degree. 



The Phenomenon of Dimorphism. 



This phenomenon was first recognised in the fossil nummu- 

 lites which abound in the marine deposits of the Eocene period, 

 and are represented by a single species living at the present day. 



