part 3] STUUCTURE OF THE FORAMIXIFEEAL SHELL. 205 



is formed of fibrils sometimes with longitudinal optic axes (fig. 3), 

 at others with transverse optic axes. How various the disjDosition 

 of the axes may be is well illustrated by comparing iigs. 3 & 4 

 with tig. 5. 



Turning now to a radial vertical section, we meet with the same 

 composite structure in all parts of the skeleton. In the fibrils of 

 the pillars the optic axis is sometimes predominately longitudinal, 

 but just as often (or more often) transverse, and in the latter case 

 often perpendicular to the plane of the section. When transverse 

 in the plane of the section it represents those fibrils with trans- 

 verse axes which are cut across in a horizontal section, and when 

 transverse perpendicular to the plane it represents similar fibrils 

 which lie on the plane of a horizontal section. It is scarcely 

 necessary to point out that a transverse optic axis may be 

 orientated at any angle in a plane at right angles to the length of 

 its fibril, and it is jDrobable that they are so orientated in pillars 

 composed of fibrils with transverse optic axes. 



The concentric walls in this as in horizontal sections are marked 

 with a zone of a faintly lighter appearance than the surrounding- 

 structure when examined Avitli ordinary light, and they are more 

 brilliantly illuminated between crossed nicols. In very thin sec- 

 tions they may sometimes be resolved into two layei'S, in one of 

 which the optic axes are radial and in the other concentric. The 

 former probably corresponds with the undulating band in hori- 

 zontal sections which remains dark through a complete rotation 

 between crossed nicols. 



It would appear from this very inadequate account that the 

 direction of the optic axis stands in no definite relation to the 

 arrangement of the fibrils; but, however unexpected this result may 

 appear, it is, I believe, in general harmony with the facts presented 

 by manv other organisms provided Avith a calcareous skeleton. It 

 finds additional illustration among the Biachiopoda ; the shells of 

 such species as I have examined {IWehratida maxillata, Ornifh- 

 opsis digona, HJii/nchoneUa concinna, and Spiriferiua ivalcotfi) 

 consist of fibres of calcite running side by side more or less pai-allel 

 to the surface, and these, though in close juxtaposition, show an 

 even greater independence in the direction of the optic axis than 

 we have already met with in Orhitolifes : for in that organism fibres 

 with similarly directed optic axes are associated in groups, while in 

 the Brachiopoda there is no fellowship of this kind, each fibre is a 

 laAv unto itself. 



We have now to consider other tonus, and will begin with the 

 Miliolidffi. In young specimens of ^pirilocuUna obtained from 

 Funafuti (^S^;. tenuis.^) the wall is remarkably thin, and so finely 

 granular that it requires a No. 9 objective (Fuess) to resolve it; 

 in consequence it is transparent and almost colourless, only faintly 

 tinged with brown by transmitted light, and only just discei-nibly 



