534 



THE LOGARITHMIC SPIRAL 



[CH. 



We learn several interesting things from this short table. We 

 see, in the first place, that where each whorl is about three times 

 the breadth of its neighbour and predecessor, as is the case in 

 Nautilus, the constant angle is in the neighbourhood of 80° ; and 

 hence also that, in all the ordinary Ammonitoid shells, and in all 

 the typically spiral shells of the Gastropods*, the constant angle 

 is also a large one, being very seldom less 

 than 80°, and usually between 80° and 

 85°. In the next place, we see that with 

 smaller angles the apparent form of the 

 spiral is greatly altered, and the very fact 

 of its being a spiral soon ceases to be 

 apparent (Figs. 271, 272). Suppose one 

 whorl to be an inch in breadth, then, if 

 Fia. 271. the angle of the spiral were 80°, the 



* For the correction to be applied in the case of the lielicoid, or "turbinate" 

 shells, see p, 557. 



