U THE FORAMINIFEEA 



early chambers ; and the position of the aperture of 

 each chamber, which subsequently becomes a stolon 

 connection, determines the axial direction of growth 

 for the succession of chambers. A lageniform 

 chamber with the aperture arranged centrally would 

 be followed by a rectilinear series of chambers, each 

 connected by a central stolon, and this would give 

 rise to a form like Nodosaria. 



There is every gradation between the rectilinear 

 and the spiral mode of growth, and an alteration in 

 the plan often occurs at a later stage of the growing- 

 shell, some genera embracing the two kinds, as 

 Penerojjlis, which in several of its species com- 

 mences with a spiral followed by a straight series of 

 chambers. 



Spirally formed shells present many modihca- 

 tions, the simplest consisting of a coiled tube wound 

 on the same plane, as in Sj/iriUuia. Or the tube 

 may be wound in a conical manner, as in Iiicolutuia. 

 Segmented spiral shells, when coiled in one plane, so 

 that in the peripheral aspect they are equilateral, are 

 termed nautiloid ; when the shell is more or less 

 conical, the primordial chamber occupying the apex, 

 the shell is trochoid or turbinoid. The whorls in the 

 latter instance are only visible on one face, the 

 superior. The coils of the spiral may be either 

 evolute, as in Oj^erculiiia, which has the whorls 

 entirely exposed on both surfaces, or involute, as in 

 PolijiitoDiella or Cristellaria, in which the shell is 

 covered up at each successive turn, usually only the 

 last whorl being visible. In the last-named genus, 



