122 J. J. LISTER. 



regarded as a "wild growing closing-in chamber," and it 

 appears probable that this is the correct interpretation. The 

 Olobigerina chambers, with their radiating spines, are probably- 

 invested with vacuolated protoplasm before the Orbulina 

 chamber is formed, as the Orbulina shell is afterwards. When 

 the growth of the shell has reached a certain stage by the 

 addition of chamber to chamber on a spiral plan, a spherical 

 chamber is formed concentric with the surface of the investing 

 protoplasm. This may completely enclose the chambers already 

 formed, or parts of these may, in rare instances, project beyond 

 it. At a later stage a second and a third shell may be added 

 while the contained chambers are absorbed. 



[Since this was written Rhumbler has published a pre- 

 liminary account (28 c) of his investigations of Orbulina, in 

 which he arrives at the same conclusion with regard to the 

 relation of the Orbulina shell to the contained chambers.] 



The formation of such a large terminal chamber, as Major 

 Owen pointed out, is not peculiar to this form. As mentioned 

 above, in specimens of Gymbalopora taken at the surface, the 

 spire of small chambers is completed by a large inflated chamber 

 far exceeding in volume the rest of the chambers taken 

 together. 



If this view of the relation of the Orbulina shell to the 

 enclosed chambers is correct, it is clear that the idea that the 

 empty Orbulina shell, and that enclosing the Olobigerina cham- 

 bers correspond respectively to the megalosphere and microsphere 

 of other Foraminifera, is untenable. As M. Schlumberger has 

 done so much to show, it is to the first formed chambers that we 

 must look for the characters which distinguish the two forms, and 

 the first formed chambers are surely those, as in all other cases, 

 which occupy the apex of the spire. It follows that, unless it 

 is shown that other characters distinguish the forms, only those 

 specimens of Orbulina in which the contained chambers are 

 complete can furnish evidence of dimorphism. 



With regard to the relationship of the free Olobigerina shell 

 to Orbulina, I have very little evidence to offer. The first 

 chamber of the free Olobigerina is about the same size as that 



