CH. XI] THE EQUIANGULAR SPIRAL 749 



of a snake, in the coils of a cuttle-fish's arm, or of a monkey's or 

 a chameleon's tail. 



Fig. 347. The shell of Nautilus pompilius, from a radiograph: to shew the 

 equiangular spiral of the shell, together with the arrangement of the internal 

 septa. From Green and Gardiner, in Proc. Malacol. Soc. ii, 1897. 



Among such forms as these, and the many others which we 

 might easily add to them, it is obvious that we have to do with 

 things which, though mathematically similar, are biologically 



