8 



In sections taken in a plane perpendicular to this, that is, 

 perpendicular to the surface of the shell and tangential to 

 a line of growth, it is evident that in the centre of the 

 section the laminae must lie in planes approximately 

 parallel to the surface, since, owing to their upward 

 bending near the surface of the shell, they must lie at one 

 point in planes perpendicular to that surface. So here the 

 section is marked out into irregular areas representing 

 small portions of the bent laminae. In any two such 

 contiguous areas the directions of the fibres are at right 

 angles to each other. Further, since the laminae are very 

 thin, several are superposed in the thickness of the section, 

 and in any one such area, by focussing, a system of parallel 

 lines crossing each other at nearly a right angle may be 

 easily seen. 



The structure of the shell is greatly complicated by the 

 sculpturing at the margin. Once the formation of the 

 definitive ribs and grooves has been initiated, the deposi- 

 tion of successive laminae proceeds upon the surface so 

 laid down, and so at the edge of the shell the surface of a 

 lamina is a very irregular one. Further back from the 

 margin, as one observes in a vertical section tangential to 

 the edge, the laminae are very regularly crumpled, the 

 contour of a single lamina being concentric with that of 

 the internal surface of the shell at its extreme margin. 

 It is obvious that this arrangement causes great irregu- 

 larity in the appearance presented by a vertical section 

 made with the intention of passing perpendicular to the 

 shell edge, for it is difficult or impossible to make the 

 section pass exactly through a rib or hollow without cut- 

 ting, in some part, the margin of the rib where the planes 

 of the laminae are approximately perpendicular to the 

 surface and margin of the shell. This causes the coarse 

 pseudo-prismatic appearance observed in a vertical section 



