Appendix 365 



subject to the same fundamental laws. I have 

 before my eyes some species of the genus 

 Terebra, from New Caledonia. They are ex- 

 tremely tapering cones, attaining almost nine 

 inches in length. Their surface is smooth and 

 quite plain, without any of the usual ornaments, 

 such as furrows, knots or strings of pearls. The 

 spiral edifice is superb, graced with its own 

 simplicity alone. I count a score of whorls 

 which gradually decrease until they vanish in 

 the dehcate point. They are edged with a 

 fine groove. 



I take a pencil and draw a rough generating 

 line to this cone ; and, relying merely on the 

 evidence of my eyes, which are more or less 

 practised in geometric measurements, I find that 

 the spiral groove intersects this generating line 

 at an angle of unvarying value. 



The consequence of this result is easily 

 deduced. If projected on a plane perpendicular 

 to the axis of the shell, the generating lines of 

 the cone would become radii ; and the groove 

 which winds upwards from the base to the 

 apex would be converted into a plane curve 

 which, meeting those radii at an unvarying 



