XI] THE SEPTA OF AMMONITES 847 



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

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

 more or less circular arcs — looking just like 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 few drops of milk 

 upon a greasy plate, or of oil upon an alkaUne solution*; they are 

 what Charles Tomlinson called "cohesion figures." 



Fig. 421. Suture-line of a Triassic Ammonite {Pinacoceras). From Zittel. 



We must not forget that while the nautilus and the ammonite 

 resemble one another, and are mathematically identical in their 

 spiral curves, they are really very different things. The one is an 

 external, the other an internal shell. The nautilus occupies the 

 large terminal chamber of the many-chambered shell, and "Still 

 as the spiral grew. He left the past year's dwelling for the new." 

 But even the largest ammonites never contained the body of the 

 animal, but lay hidden, as Spirula does, deep within the substance 

 of the mantle. How the comphcated septa and septal outHnes of 

 the ammonites are produced I do not knowt. 



We have very far from exhausted, we have perhaps little more 

 than begun, the study of the logarithmic spiral and the associated 

 curves which find exemplification in the multitudinous diversities 

 of molluscan shells. But, with a closing word or two, we must 

 now bring this chapter to an end. 



* "The Fimbriae, or Edges, appeared on the Surface like the Outlines of some 

 curious Foliage. This, upon Examination of them, I found to proceed from the 

 Fulness of the Edges of the Diaphragms, whereby the Edges were waved or plaited 

 somewhat in the manner of a Ruff" (R. Hooke, op. cit.). 



t In certain rare cases the complicated sutural pattern of an ammonite is found 

 upside down, but unchanged otherwise. Cf. Otto Haas, A case of inversion of 

 suture hues in Hysteroceras, Amer. Jl. of Sci. ccxxxix, p. 661, 1941. 



