Umbilicus (mollusk)
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In mollusks, the umbilicus of a shell is the axially aligned hollow cone within the whorls of a coiled gastropod shell, where no columella has been formed. This is the hole around which the inner surface of the shell is coiled. In species with a wide, open umbilicus, such as the Heath Snail (Helicella itala), the spiral of the whorls can be perceived from the posterior end of the shell.
The umbilicus can vary from very narrow and punctured, as found in Trichia unidentata, to wide and shallow, such as the deep and wide depression in the Rounded Snail (Discus rotundatus). Shells with a conspicuous umbilicus are always orthostylic, i.e. with a poorly developed columella.
Sometimes there is a dimple or funnel-shaped depression, known as the umbilical region or the umbilical field, next to or at the basal hollow of the columella, when the walls of successive whorls are not closely wound against each other.
