82 PHYTOPHAGA. 



the inner surface, as in this genus, in which the horny 

 part is very thin and scarcely visible, except where 

 the shelly coat is very thin, as at the flexible edge. 



These animals absorb the septa which separate the 

 whorls of the spire, when they have arrived at their 

 full size, so as to allow more room for the spiral 

 body, without increasing the size of the shell ; and 

 this can be done without endangering the strength of 

 the shell, as only a very small part of the whorl is 

 exposed on the surface. A similar absorption is to 

 be observed in many AuriculidcB^. and to a less extent 

 in the Cones, where the septa are only reduced in 

 thickness. (See Phil Trans. 1833, p. 798.) 



This absorption is only superficial, and produced 

 by that portion of the surface of the mantle which lies 

 close to it, and is not to be confounded with the ab- 

 sorption of the bones of vertebrated animals, where it 

 is produced by vessels which ramify in the substance 

 of the bone, and which are accompanied by other 

 vessels to replace with new portions the part which 

 has been removed. 



The apices of the spires of these shells are some- 

 times eroded ; those are more so which live in stag- 

 nant or nearly stagnant waters. The late Mr. Sowerby 

 (Mm. Conch, iv. 49.) supposed that this was produced 

 by " some acid developed during the fermentation 

 of vegetable matter in marshes or at the bottoms of 

 the rivers." Others, who were not aware how the 

 animals walked, have said that this erosion of the apex 

 was produced by the animal rubbing it against the 

 ground in progression ; explaining also the erosion of 

 the umbones of the Uniones in the same manner. 



