A Field Guide to the Snails of Lord Howe Island

153 Glossary aperture - the opening of a shell through which a snail can extend its body. circum-umbilical - around the umbilicus. columella - the central axis of a shell, only visible at the inner edge of the aperture. congeneric - two or more species that belong to the same genus. conical - with a cone-shaped shell. determinant growth - where the shell ceases to grow larger and add new whorls at maturity. Often the shell instead becomes thickened with increasing age. dextral - shell coiled in a right-handed or clockwise direction. discoidal - with a disc-shaped shell. endemic - only occurring in a certain geographic region (in the context of this guide, Lord Howe Island). genus - a taxonomic category defining a group of closely related species. globose - with a globe-shaped shell. hermaphroditic - where both male and female reproductive organs are present in a single individual. imperforate - with a closed umbilicus. indeterminant growth - where the shell continues to grow even after a snail reaches maturity. introduced - a species that is not naturally occurring in a particular geographic region, but has been brought there by human means. Other terms include exotic, alien, non-indigenous. keeled - with a strong ridge running around the periphery of each whorl of a shell. lip - the outer edge of the aperture. operculum - a small door that can be used to seal the aperture of a shell. oviparous - a species that lays eggs. ovoviviparous - a species that incubates eggs internally within the reproductive system, so that when laid they will hatch immediately or within a very short time.