SEA-FISHERIES LABORATORY. 185 
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 lamine must le ‘in planes approximately 
parallel to the surface, since, owing to their upward 
bending near the surface of the shell, they must he 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 lamine. In any two such 
contiguous areas the directions of the fibres are at right 
angles to each other. Further, since the lamine are very 
thin, several are superposed in the thickness of the section, 
and in any one such area, by focussing, a system of parailel 
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 lamine 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 lamine 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 lamine are approximately perpendicular to the 
surface and margin of the shell. This causes the coarse 
pseudo-prismatic appearance observed in a vertical section 
