106 THE FORAMINIFERA 



The megalospheric form begins in a structure called by Carpenter 

 the primitive disc (Figs. 36, 37, A, and 38). It consists of (1) 

 the megalosphere, which is pear-shaped (about 107 p. in mean 

 diameter) ; (2) a spiral passage leading from the megalosphere, 

 and opening into (3) a large crescent-shaped chamber, one horn 

 of which extends round one side of the megalosphere, and the 

 other along the outer side of the spiral passage. It results from 

 this arrangement that the outer wall of the latter forms a parti- 

 tion (p) disposed perpendicularly to the flattened surface of the 

 primitive disc, and separating the spiral passage from the crescentic 

 chamber. The partition ends in a free border. The spiral passage 



-r 



Fro. 



Diagram representing the transition from the simple ("marginalis') to the complex 

 (" complanata ") type of structure in the growth of a sub-typical individual of Orbitolites com- 

 planata. The primitive disc and half the test of a megalospheric form are represented in 

 section. The letters P D are placed beneath the centre of the primitive disc, m, part of test 

 formed on the marginalis type ; d, that formed on the duplex type ; /, that formed on the 

 type of a fossil form of 0. complanata ; c, that of the typical 0. complanata ; ac, annular canals ; 

 cs, columnar spaces ; mp, marginal pores ; r, radial canals ; sc, superficial chamberlets. (After 

 Carpenter, but modified.) 



with the crescentic chamber together compose the circumambient 

 chamber of Carpenter. The whole of the peripheral wall of the 

 circumambient chamber is perforated by pores opening into the 

 innermost chamberlets, which are thus disposed in a complete 

 annulus from the beginning. In some primitive discs there is a 

 single row of pores at the margin, in others there are two or 

 three rows. In the latter case the three-layered arrangement of 

 chamberlets characteristic of 0. complanata is assumed directly ; 

 while in the former the region of the test immediately surround- 

 ing the primitive disc may present varying degrees of development. 

 In some (Fig. 38) the rings of chamberlets are at first in single 

 series, arranged on the marginalis type, and they are succeeded by 

 annuli on the duplex type, the three -layered character being 

 ultimately assumed. In others the arrangement begins on the 

 duplex type. Here, again, we have examples of initial polymorphism. 

 On comparing the primitive disc of 0. complanata with the 

 centre of the tests of the megalospheric forms of the other species, 

 it appears that the crescent-shaped chamber of complanata may be 



