XI] 



OF SEPTA 



581 



curvature is also constantly altering. But in the case of Nautilus, 

 there are other aspects of the phenomenon, which we can illustrate, 

 but onlv in part, in the following simple manner. Let us imagine 



Fig. 304. Section of Nautilus, shewing the contour of the septa in the median 

 plane: the septa being (in this plane) logarithmic spirals, of which the shell- 

 spiral is the evolute. 



a pack of cards, in which we have cut out of each card a similar 

 concave arc of a logarithmic spiral, such as we actually see in the 

 median section of the septum of a Nautilus. Then, while we hold 

 the cards together, foursquare, in the ordinary position of the 



