844 THE EQUIANGULAR SPIRAL [ch. 



But while the outline of the septum in median section is simple 

 and easy to determine, the curved surface of the septum in its 

 entirety is a very complicated matter, even in Nautilus which is 

 one of the simplest of actual cases. For, in the first place, since 

 the form of the septum, as seen in median section, is that of a 

 logarithmic spiral, and as therefore its curvature is constantly 



Fig. 419. C&,Bi oi the mteviov oi Nautilus-, to shew the contours of 

 the septa at their junction with the shell-waU. 



altering, it follows that, in successive transverse sections, the curva- 

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

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

 only in part, in the following simple manner. Let us imagine a ^ack 

 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 pack, we have 

 a simple ''ruled" surface, which in any longitudinal section has the 



