98 PALEONTOLOGY 



last collectively form the spire. Lines drawn at a 

 tanger.t to opposite sides of the spire will meet beyond 

 the apex at an angle of 11, which is the spiral angle of 

 this particular species. The spiral line which marks the 

 visible junction of one whorl with the next is called the 

 suture. The side of each whorl forms an apparently con- 

 vex curve, but this is seen under a lens to be made 

 up of eight concave curves, the ends of which correspond 

 to seven more or less conspicuous ridges which run 

 spirally round the shell from apex to base, bearing on 

 them a vast number of fine and close-set tubercles. 

 Between and parallel to these ridges are much finer lines, 

 from four to twelve in each interval. These are all 

 crossed by very fine growth-lines, parallel to the margin 

 of the aperture. 



The aperture, or mouth, of the shell is oval in shape. 

 When the shell is placed in the usual position chosen for 

 figures of gastropods apex upwards* the mouth is 

 seen on the right-hand side of the base (this being a 

 right-handed spiral or dextral shell, as was also Emav- 

 ginula}. The margin of the aperture is called the 

 peristome ; it is divided into inner lip (near the middle line) 

 and outer lip. Living species of Tuvvitella have a horny 

 operculum, which fits into the aperture when the animal * 

 is completely withdrawn into the shell ; but this, being 

 horny, is not found fossil. 



If a vertical section is cut through the axis of the 

 spiral, it is seen that the inner faces of the whorls are 



* This is the accepted position in English works, but in French 

 works it is usual to place the apex downwards. 



