194 PKOF. w. J. soLLAS OX saccam:mixa. caeteei, [vol. Ixxvii. 



perhaps colloidal origin, assumes the rounded contours of organic 

 form that have proved misleacling to more than one distinguished 

 observer. In this case it is responsible, both for the ' scars ' and,. 

 as we shall see directly, for the labyrinthine structure. 



The isolated chambers of the fossil, rarely connected together,, 

 are thickly scattered through the limestone, so that if the minerals 

 now filling their interior were removed the rock would be rendered 

 highly vesicular, in some cases as much as from a third to a half of 

 its volume would then be occupied by empty space. 



An examination of thin slices under the microscope reveals con- 

 siderable diversity in the mineral changes which have followed on 

 the death of the animal. 



In the best-preserved examples the wall of the chamljer is a very 

 thin calcareous shell of uniform thickness, presenting a smooth 

 surface both within and without. It consists of an irregular 

 mosaic of minute calcite-crystals, which by its comparative pmity 

 and transparency, in striking contrast with the dark dusty appear- 

 ance of the surrounding matrix, is well defined on the exterior. 

 On the interior also it is well defined, partly by washed-in matrix — 

 which in rare examples completely fills the test, but is more 

 "usually present only in patches — , partly by a growth of very minute 

 calcite-crystals, which are often more nearly colourless than the 

 wall, but appear darker in mass owing to their more abundant 

 interspaces and in consequence the greater loss of light by internal 

 reflection. In some cases a thin layer of black carbonaceous 

 granules with associated granules of iron pyrites lies next the inner 

 surface, and marks it off in bolder outline (PI. VII, figs. 10, 18). 

 L. Rhumbler^ has called attention to the presence of granules 

 of pyrites in the chambers of the recent species of Saccammina 

 (S. sphferica) as well as of several species of perforate forms, and 

 remarks that it may sometimes be seen in the decomposing 

 sircode still present in the chambers of dead foraminifera. He 

 explains its formation as due to the reduction of ferrous sulphate 

 by organic matter. This view is in harmony with the intimate 

 association of carbonaceous matter and pyrites in our specimens. 



Owing to the excellent definition of the boundary of the chamber- 

 wall on both sides, its thickness may be measured with considerable 

 accuracy. This amounts in the great majority of cases to O'Oo mm. ; 

 but it may be less, sometimes falling to 0"02 mm. : or it may be 

 luore, as much occasionally as 0*08 mm. 



Passing now to the mineral infilling of the chamber, we find 

 that in the sim])lest case its cavity is completely occupied by a 

 coarse mosaic of calcite which has grown from the wall inwards. 

 Some of the crystals of the fine mosaic which forms the Avail 

 have shared in this secondary growth, and project inwards a little 

 beyond the inner boundary, which thus assumes an appearance 



^ ' Beitrage zur Kentniss der Etdzopoden, II.' Zeitschr. i. Wissensch. Zool.. 

 vol. Ivii (1894) pp. 433-617, in particular p. 571. 



