846 THE EQUIANGULAR SPIRAL [ch. 



We have other sources of complication, besides those which are 

 at once .introduced by the sectional form of the tube. For instance, 

 th'e siphuncle, or little inner tube which perforates the septa, exercises 

 a certain amount of tension, sometimes evidently considerable, upon 

 the latter: which tension is made manifest in Spirula (and slightly 

 so even in Nautilus) by a dip in the septal floor where it meets the 

 siphuncle. We can no longer, then, consider each septum as an 

 isotropic surface under uniform pressure; and there may be other 

 structural modifications, or inequalities, in that portion of the 



Fig. 420. Ammonites Sowerbyi. From Zittel. 



animal's body with which the septum is in contact, and by which 

 it is conformed. It is hardly likely, for all these reasons, tha»t we 

 shall ever attain to a full and particular explanation of the septal 

 surfaces and their sutural outlines throughout the whole range of 

 Cephalopod shells; but in general terms, the problem is probably 

 not beyond the reach of mathematical analysis. The problem might 

 be approached experimentally, after the manner of Plateau's experi- 

 ments, by bending a wire into the complicated^ form of the suture-line, 

 and studying the form of the liquid film which constitutes the 

 corresponding surface 'minimae areae. 



In certain Ammonites the septal outline is further complicatec" 

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



