152 



MOLLUSCA. GASTEROPODA. 



short, or depressed. The angle of the spire (spiral angle) 

 will consequently also vary; this is measured by lines 

 drawn from the apex to the base of the shell on opposite 

 sides of the exterior of the whorls. The coiling of the 

 shell is usually dextral ; that is when the apex of the 

 shell is pointed away from the observer (as in fig. 47), the 

 aperture will be on the right hand-side ; in a few cases it 

 is sinistral, when the aperture will be on the left. 



FIG. 47. Longitudinal section of Triton corrugatus. The upper part of 

 the spire has been partitioned off many times successively, a, apex; 

 su, suture ; S, spire ; L, outer lip of the aperture ; ac, anterior 

 canal; pc, posterior canal. (From Woodward.) 



Frequently the inner parts of the whorls coalesce, and 

 form an axial pillar extending from the apex to the base 

 of the shell (fig. 47), and known as the columella. In 

 other cases the inner parts do not fuse and in the place of 



