Oit THE LIFE-aiSTORY OF THE FORAMlNlFERA. 117 



fact that the thickness of the shell at the centre is only one- 

 third of that of the megalospheric form. However, in alluding 

 to the question of dimorphism, Brady considered that the 

 evidence of Orbitolites supported the view suggested by MuNiER- 

 Chalmas that the microspheric form was a modification of the 

 megalospheric. 



In 1890, ScHLUMBERGER (35) described Adelosina polygonia, 

 a new species of the Miliolidae. From the figures it appears 

 that the dimensions of the megalosphere and microsphere of 

 this species are, in particular instances, 216 /i and 25 x 18 /i 

 respectively. The plans of growth of the two forms are difi'er- 

 ent throughout, the megalospheric forms having the chambers 

 arranged (in 99 cases out of 100) in a triangular, while the 

 microspheric forms have them arranged in a quadrangular 

 figure. 



Among the specimens examined the frequency of occurrence 

 of the megalospheric and microspheric forms was as 8 to 1. In 

 fully-formed shells the microspheric form is slightly smaller 

 than the megalospheric (microspheric form 1'4 mm., megalo- 

 spheric form 1-5 mm. in diameter). Young specimens of the 

 microspheric form occur. It is pointed out that in this case 

 the view suggested by Munier-Ohalmas, and adopted by the 

 author, that the microspheric is a development of the megalo- 

 spheric form, is clearly untenable. 



In May, 1893, Van den Broeck (5) published his "Etude 

 sur ie dimorphisme des Foraminiferes," in which the relation- 

 ship of the two forms is discussed. 



In this work the author reviews the evidence on the subject 

 of dimorphism, and the reasons for regarding the two forms as 

 distinct from their origin are urged with much force. It is 

 shown that the difference in the size of the central chamber 

 indicates a difference in the reproductive processes by which the 

 two forms originate. 



In speculating as to the nature of this difference Van den 

 Broegk puts forward the view, which, as is stated, had pre- 

 viously been suggested by DoLLFUS (' Annuaii-e Gdologique,' 

 1890, p. 1099), that the megalospheric young are the result of 



