RHIZOPODA. 15 



section of the order Helicostega of D'Orbigny. 

 It has since, however, been shown that many 

 Foraminiferous shells which commence their 

 growth upon the spiral planj e. g. Gristellaria, 

 ultimately assume a straight form, so as to resem- 

 ble Nodosaria {fig. 3, 6, h'). Examples of nau- 

 tiloid Ehizopods may be found in Polystoraella 

 {fig. 3, c), and in the well-known fossil Nummu- 

 lites {fig. 3, d). 



In Cassidulina {fig. 3, e) and its allies, each of 

 the chambers of which the spiral shell consists is 

 furnished with two surfaces of unequal size, 

 which are alternately presented to opposite sides 

 of the shell. Other modifications of the Polv- 

 thalamous structm'e are presented by Textularia 

 {fig. 3,/), and Miliolina {fig. 3, g), the nature 

 of which will best be understood when we come 

 to consider the mode of growth of the shell. 



g. I^ltructure of the §hell in Foraniiwifera. 



— The structure of the shell has been ably inves- 

 tigated by Drs. Carpenter, Williamson, and others, 

 whose combined researches have proved that its 

 many complex forms all result from continuous 

 processes of gemmation, and that the several 

 varieties of these are dependent on corresponding 

 variations in the plan upon which this gemmation 

 is conducted. For all the multilocular Ehizopods 

 consist at first of but a single chamber. Should 

 the latter put forth another chamber similar to 

 itself, and in a direct line with the axis of its body, 

 this process, repeated several times, would give 

 rise to such a form as Nodosaria, in which 

 the original orifice of the first chamber serves as 

 an aperture communicating between it and the 



