582 THE LOGARITHMIC SPIRAL [ch. 



pack, we have a simple "ruled" surface, which in any longitudinal 

 section has the form of a logarithmic spiral but in any transverse 

 section is a straight horizontal line. If we shear or shde the 

 cards upon one another, thrusting the middle cards of the pack 

 forward in advance of the others, till the one end of the pack is 

 a convex, and the other a concave, ellipse, the cut edges which 

 combine to represent our septum will now form a curved surface 



Fig. 305. Cast of the interior of Nautilus : to shew the contours of 

 the septa at their junction with the shell -wall. 



of much greater complexity; and this is part, but not by any 

 means all, of the deformation produced as a direct consequence 

 of the form in Nautilus of the section of the tube within which 

 the septum has to lie. And the complex curvature of the surface 

 mil be manifested in a sinuous outline of the edge, or line of 

 attachment of the septum to the tube, and will vary according 

 to the configuration of the latter. In the case of Nautilus, it is 

 easy to shew empirically (though not perhaps easy to demonstrate 



