818 THE EQUIANGULAR SPIRAL [ch. 



From this table, by interpolation, we may easily fill in the 

 approximate values of a, as soon as we have determined the apical 

 angle ^ and measured the ratio R\ as follows: 



We see from this that shells so different in appearance as Cerithium, 

 Solarium, Natica and Planorbis differ very httle indeed in the 



• magnitude of the spiral angle a, that is 

 to say in the relative velocities of radial 

 and tangential growth. It is upon the 

 angle /? that the difference in their form 

 mainly depends. 



The angle, or rather semi-angle (/?), of 

 the tangent cone may be taken as 90° 

 in the discoid shells, such as Nautilus 

 and Planorbis. It is still a large angle, 

 of 70° or 75°, in Conus or in Cymba, 

 somewhat less in Cassis, Harpa, Dolium 

 or Natica ; it is about 50° to 55° in the 

 various species of Solarium, about 35° 

 in the typical Trochi, such as T. niloticus 

 or T. zizyphinus, and about 25° or 26° in 

 Scalaria pretiosa and Phasianella bul- 

 - hides', it becomes a very acute angle. 



Fig. 397. TerebramacuJutal.. ^^ ^^o ^ ^^o ^ ^^ ^^^^ j^^^^ -^ ^^^^.^^^ 



Turritella or Cerithium. The British species of ' Fusus ' form a series 

 in which the apical angle ranges from about 28° in F. antiquus, 

 through F. Norvegicus, F. berniciensis, F. Turtoni, F. Islandicus, 

 to about 17° in F. gracilis. It varies much among the Cones; and 

 the costly Conus gloria-maris, one of the great treasures of the 

 conchologist, differs from its congeners in no important particular 



