part 3] aind structure of the foraminieeral shell. 203 



parallel to a set of edges. ^ Such a felt-like intei'growth occurs in 

 the shell of Orhitolites; but, in all cases in which I could ac- 

 curately observe the extinction of an individual crj'stal, it was- 

 found to be rigidly straight, and thus no explanation is to be 

 sought in this direction. It may be added that the transverse 

 direction of the optic axis has been determined in individual 

 crystals. 



All difficulty disappears, however, when we recall that we are 

 dealing with organic products, the outer form of which may be 

 completely independent of their crys^talline structure. The spicules- 

 of calcareous sponges offer an excellent illustration. This was 

 shown long ago in describing the phenomena of extinction pre- 

 sented by them ^ ; and it may now be added that, Avhile the optic 

 axis of a calcareous spicule which gives straight extinction is in 

 some cases directed parallel with the length, in as many others it 

 lies transverse to it. 



Of the independence of crystalline structure and organic form 

 we shall presently discover in SpiriUina an equall}^ interesting 

 example. 



Starting now from the fundamental fact that the optic axis is a& 

 often transverse to the axis of a fibril as it is longitudinal, I may 

 proceed to complete the description of the intimate structure of 

 the shell, which is more complex and various than might be sup- 

 posed. I will commence with an account of the structure displayed 

 in horizontal sections. 



In all cases abundant fibrils and granules occur throughout the 

 structure with their optic axes directed perpendicularly to the plane 

 of the section. They are especially concentrated, however, in certain 

 areas, as, for instance, the layer already mentioned as giving rise ta 

 the curved arms of the black cross shown in fig. 1 (p. 204). In some 

 cases the crown of an arch is formed mainly of three layers, of 

 which that just referred to is the middle one separating an upper 

 la\'er {a, fig. 2) in which the optic axes are, for the greater part, 

 directed across the length of the fibrils (transverse optic axes), 

 from a low^er one (h, fig. 2) in which they coincide in direction 

 with its length (longitudinal optic axes). 



Such an arrangement is, however, by no means constant, or 

 even predominant. The relation of the 0])tic axes to the fibrils 

 seems indeed to be governed by no rule. Thus sometimes, as re- 

 presented diagrammatically in figs. 3 & 4, the pillars consist mainly 

 of fibrils with ti-ansverse optic axes, at others, as in fig. 5, of fibrils 

 with longitudinal optic axes, or again, as in fig. 4, of a core of 

 fibrils with transverse optic axes surrounded by a wall of fibrils- 

 with longitudinal optic axes. Similarly, the floor of the chambers 



^ J. Johnston, H. E. Merwin, & E. D. Williamson, op. cit. p. 538. 



^ W. J. Sollas, ' On the Physical Characters of Calcareons & Siliceous 

 Sponge-Spicnles & other Strnctures ' Sci. Proc. Eoy. Dublin Soc. v-. s. vol. W 

 (1885) p. 374. 



