CLASSIFICATION at 
is also more frequently than not ornamented with 
either spiral lines or ridges running in the direction 
of growth, or with transverse markings coinciding 
with the lines of growth or with both (Plate V., 
Fig. 11-17). 
The external spines that come in the way of the 
growth of the shell as whorl is added to whorl are 
dissolved in some way, and removed by the animal. 
In some cases also the internal walls of the whorls 
and the columella are similarly removed to make 
more room for the growing creature (¢.g., Neriia, 
Theodoxis, Cyprea, Conys [Plate VIL., Figs. 12 and 13], 
Auricula, etc.). 
On the other hand, when the animal in the course 
of growth leaves a space in the upper whorls, this is 
either filled up with a shelly deposit (Plate XXXII., 
Fig. 6), or cut off by the formation of a wall or 
septum across the whorl (Plate VI., Fig. 2, s.m., and 
in Vermetus, etc.).* A similar partition is found 
when the apex of the spire becomes worn through 
or broken by accident, or attacked by boring Molluscs 
(Plate VII., Fig. 13), or is broken off as a regular 
successive phase of the animal’s existence (e.g. in 
Caecum [Plate VII., Fig. 14], Rumina decollata 
’ [Plate VII., Fig. 15], etc.). 
Many of the Gastropods, like the Periwinkle, 
close the mouth of the shell, on retiring into it, with 
* One, and only one, instance of this in a helicoid has been 
recorded—viz., in a species of Glypiostoma. 
