234 



MOLLUSCA. GASTEROPODA 



fig. 98) the aperture will be on the right-hand side ; in a 

 few cases it is sinistral, when the aperture will be on the 

 left. 



Frequently the inner parts of the whorls coalesce, and 

 form an axial pillar extending from the apex to the base of 

 the shell (fig. 98) and known as the columella. In other cases 



Fig. 98. Longitudinal section of THtonium corrugatum. 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.) 



the inner parts do not fuse, and in the place of the columella 

 there is left a tube-like space, extending from the base of 

 the shell a greater or less distance towards the apex ; this 

 space, which opens at the base of the shell, is called the 

 ■umbilicus. When there is a columella the shell is often 



