CLASSIFICATION 21 



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. n-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 {e.g., Nerita, 

 Theodoxis, Cyprcea, Coitus [Plate VII., 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 

 C cecum [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 Glyptosioma. 



