Within the mantle, most land slugs have the remnants of what was in the evolutionary past a larger, external shell. Usually this remnant is either a small, thin, shell-like plate or a collection of calcareous (chalky) granules. The Kerry slug has an internal shell or shell plate that resembles those found in land slugs of the genus Limax; it is ovoid, solid and chalky with a transparent conchiolin (horny) base. The shell plate is usually convex above and concave beneath and has some indistinct, concentric lines of growth. According to Godwin-Austen, the exterior of the shell plate is covered with a thin, transparent protein layer called the periostracum and with the nucleus—the first part to form—situated near the front. In young Kerry slugs the shell is very thin and convex, abruptly cut off behind, and with an extremely thin layer that projects in front and contains minute granules.
