164 J. J. LISTER. 



distinguished as Megalospheric and Microspheric. The contrast 

 in size between the megalosphere and microsphere is much 

 greater in some species than in others. 



In Biloculina depressa, d'Orb., according to Schlumberger, 

 the diameter of the megalosphere is between 200 and 400 fi, 

 while the mean diameter of the microsphere is 20 /a. 



In Polystomella crispa (Linn.), the megalosphere varies, in 

 the specimens in which I have measured it, from 165 to 85 /* in 

 diameter, and the microsphere from 13 to 65 /j,. 



In Rotalia Beccarii (Linn.), the difference is less. Among 

 five examples of the megalospheric form the diameter of the 

 megalosphere varied from 65 to 37 /i; the diameter of the 

 microsphere in one individual was 13 /z. 



In the allied form, Calcarina hispida, Brady, among the 

 twelve examples the diameter of the megalosphere varied from 

 58 to 42 fM, and that of the microsphere in two examples from 

 14 to 12 fi*. 



In the description above given of Truncatulina, it was shown 

 that, while the megalosphere is usually considerably larger than 

 the microsphere, it may approach it closely in size. Possibly 

 there are genera of Foramiuifera in which the central chambers 

 of the two forms are of the same size. In such cases, analogy 

 with the types above described would lead us to look for differ- 

 ences in the nuclei by which they might be distinguished. It 

 cannot however be assumed that all forms of Foraminifera are 

 dimorphic. 



(6) The shape and mode of growth of the chambers suc- 

 ceeding the megalosphere and microsphere. 



Schlumberger has called attention to the very interesting 

 fact, that in many species among the Miliolidae the chambers 

 immediately succeeding the central one are arranged on the 

 biloculine plan in the megalospheric form, and on the quinque- 

 loculine plan in the microspheric form (woodcut, p. 114.) 



* The numbers given are not strictly comparable in the different species, for 

 while, in Biloculina, the measurements are taken from sections of the shell, in 

 the others they are from decalcified and mounted, and hence somewhat 

 shrunken, specimens. 



