584 



THE LOGARITHMIC SPIRAL 



[CH. 



a wire into the complicated form of the sutiire-hne, and studying 

 the form of the Uquid fihn which constitutes the corresponding 

 surface minimae areae. 



Fig. 306. Ammonites (Sonninia) Sowerbyi. (From Zittel, after 

 Steinmann and Doderlein.) 



In certain Ammonites the septal outline is further compUcated 

 in another way. Superposed upon the usual sinuous outline, with 

 its "lobes" and "saddles," we have here a minutely ramified, or 

 arborescent outline, in which all the branches terminate in wavy. 



Fig. 307. Suture-line of a Tiiassic Ammonite (Pinacoccras). 

 (From Zittel, after Hauer.) 



more or less circular arcs, — looking just Uke the 'landscape 

 marble ' from the Bristol Rhaetic. We have no difficulty in 

 recognising in this a surface-tension phenomenon. The figures 

 are precisely such as we can imitate (for instance) by pouring a 



