236 Mr Lister, The young of the microspheric forms [Jan. 25, 



necessary for the conduction of impulses and thinks that the 

 amount of this film varies, resulting in the variations in the state 

 of " openness " or inhibition of the reflex path. 



(2) A possible explanation of the quinqueloculine arrangement 

 of the chambers in the young of the microspheric forms of Triloculina 

 and Biloculina. By J. J. Lister, M.A., St John's College. 



In the later stages of growth of the shells belonging to the 

 genus Biloculina the arrangement of the chambers is very simple 

 (Figs. 1 — 3). They are disposed on either side of an axis, the mouth 

 being directed alternately to one and the other end of this axis, 

 and the median plane which divides any single chamber symmetri- 

 cally, is also the median plane of the whole series of the chambers 

 added during the later stages of growth. In the megalospheric 



Fig. 1. Biloculina depressa d'Orb. Megalospheric form, longitudinal section. 



28. 



Fig. 2. Transverse section of a specimen with a smaller megalosphere of the 

 same species, x 50. 



form (Figs. 1 and 2) this arrangement is followed from the 

 beginning, that is to say, the two chambers succeeding the megalo- 

 sphere and all subsequently formed chambers are so disposed. 



