86 THE ELEMENTS 



" In order to determine whether a 

 flat shell, whose volutions are laterally 

 placed, is a reversed species, we have 

 only to examine which way the volu- 

 tions turn from the apex, or the centre, 

 towards the mouth ; and if we find it 

 contrary to the motion of the index of 

 a watch, it is a heterostrophe, or revers- 

 ed shell ; and vice versa." 



A flat shell is figured in plate 3d, 

 fiV. 14th. 



" In some of the more depressed 

 species of Helix, or Nautilus, atten- 

 tion is requisite to be paid to the 

 mouth, in order to determine which is 

 really the upper side of the shell, for 

 it is on that side the spiral turns are 

 to be taken from the centre or apex ; 

 and, in most instances, this is to be 

 determined by the oblique direction 

 of the aperture to the under part, 

 where the lip rarely extends so far as 

 on the upper part. In fixed shells, 

 such as Serpula, there is no difficulty, 





