CH. Xl] 



OF VARIOUS UNIVALVES 



559 



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

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

 apical angle d 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 little 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 6 

 that the difference in their form 

 mainly depends : that is to say the 

 amount of longitudinal shearing, 

 or displacement parallel to the axis 

 of the shell. 



The enveloping angle, or rather 

 semi-angle (6), of the 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, Dohum 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 

 23retiosa and Phasianella bulloides ; it 

 becomes a very acute angle, of 

 15°, 10°, or even less, in Eulima, 

 Turritella or Cerithium. The costly Conus gloria-maris, one of the 



Fist. 285. Terebra maculata, L. 



