140 ELEVENTH REPORT. 



Glossary : 



« 



Aperture — the opening throufih which the animal extrudes itself. 



Callus — thickening of the shell, usually around aperture. 



Columella — twisted central axis of shell. 



Mantle — membrane surrounding respiratory cavity. 



Operculum — calcareous "door" to the aperture, borne on the 



animal's foot. 

 Parietal wall — wall of aperture nearest central axis Qf shell. 

 Peristome — edge of aperture. 



Subdiscoidal — flattened, spire rising but little over last whorl. 

 Umbilicus — cavity in base of shell. 

 Vitreous — glassy, very clear and transparent. 



