part 3] AXD STELX'TURE OF THE FORAMUSTIFEEAL SHELL. 195 



that has been raistakenl}^ supposed to indicate an arenaceous 

 structure. That this is a secondary character which did not exist 

 during" life, but was subsequently acquired, is shown in some cases 

 hy the presence of a thin dark line which sharply separates the 

 later growth from the original mosaic of the test : but, even 

 without this evidence, the mode of growth of the secondary crystals 

 sufficiently reveals its true nature. 



In the next case quartz contributes to the infilling of the chamber 

 (PL VII, figs. 9, 12). The mosaic of calcite extending inwards from 

 the wall may fail to reach the centre, leaving a larger or smaller 

 space which is then filled with quartz, sometimes, as seen in sections, 

 by a single crystal, sometimes by a mosaic. 



Usually, when quartz is present, it is accompanied b}^ chalcedony 

 (PI. YII, figs. 8, 14) which first attacks the wall of the chamber, 

 often completely replacing the mosaic of calcite, and then extends 

 for a greater or less distance inwards. Where it invades the wall, 

 its fibres start from centres on the inner boundary and radiate out- 

 wards ; where it fills the interior, its fibres also originate in centres 

 on the inner boundary, but radiate inwards. This inward growth 

 evidently proceeded rhythmically, zones of chalcedon}'' alternating 

 with zones of quartz, in the fashion so familiar to us in agate: 

 in some instances as many as seven of these zones may be counted 

 in a deposit 0"6 mm. thick. 



The chalcedony may be readily distinguished from the quartz in 

 ordinary light, partly by its faint yellowish-brown tint (the quartz 

 being absolutely colourless), and partl}^ by its finely granular 

 appearance and less perfect transparency. But it requires polarized 

 light to reveal the minute structure of the chalcedony in all its 

 beauty. 



In some cases the outer third of a chamber may be filled with 

 chalcedony, the middle third with calcite, and the central region 

 with quartz : or the outer zone may be formed by an inter- 

 penet]'ating growth of chalcedony and calcite, and then we have the 

 labyrinthine structure described by Brady. ^ 



In the light of this explanation it is interesting to read 

 Khumbler's ^ account of a labyrinthine structure in the wall of a 

 living species (JV^odosiiiella r/ansica), which he regards as com- 

 parable with that of Saccammina carteri. 



In concluding this description I may call attention to the 

 abundant presence in the Saccammina Limestone of thin bands of 

 calcite mosaic, curved in circular arcs, which by their structui-e, 

 thickness, and radius of curvature may be recognized as fragments 

 of the chamber-wall. Thus the organism has contributed far 

 more to the substance of the limestone than would be concluded 

 from observation of the imbroken chambers alone. 



^ H. B. Brady, op. cit., see in particular pi. i, fig's. 5 & 6. 

 ^ L. Ehiimbler, ' Die Foraminiferen der Plankton-Expedition ' Ergebnisse 

 der Plankton-Expedition der Humboldt-Stiftung, vol. iii, pt. 2 (1913) p. 452. 



