78 THE FOEAMINIFEEA 



and usually have more than two to each turn of the 

 shell. The later growth of the shell is sometimes 

 rectilinear, and, as in ArticuUiia, may closely 

 resemble the extended forms of Nuhecularia. 



PenerojjUs may have its origin in a simple form, 

 like Co7muspira, by its becoming septate. Peneroplis 

 exhibits a spiral series of chambers the septa of 

 which are numerously perforate, and the spiral 

 manner often changes in later growth to an elongate 

 straight series. The sub-genus ArcJtiacma forms a 

 link with genera having a cyclical arrangement of 

 chambers by showing a tendency towards that plan 

 of growth in the last formed portion of the adult 

 shell. 



'Peneroplis passes into Orhiculina by the sub- 

 division of its chambers into chamberlets by means 

 of vertical partitions connecting septum wdth sep- 

 tum. The last-named spirally formed genus, again, 

 forms a link with the discoidal shell of Orhitolites : 

 this genus possesses a subglobular primordial cham- 

 ber, and the succeeding chambers are distinctly 

 spiral, but usually quickly assume a cyclical (con- 

 centric) manner of growth, although in one species 

 at least a considerable part of the earlier shell is spiral. 

 The annular chambers in Orhitolites are always 

 subdivided into little subquadrate chamberlets, each 

 with its stolon aperture for the passage of the 

 sarcode into the neighbouring compartments. There 

 are often two, three, or even more tiers of annuli 

 superposed to form the entire shell, and these, in 



