This large pill-woodlouse (to 20mm) is typically slate grey, sometimes flecked with yellow, and reminiscent of the common A. vulgare. It differs in the pleon being more curved outwards, giving a 'splayed' or 'flattened' appearence, and often leaves a small gap when enrolled. The male 1st pleopods are diagnostic.
Distribution and Habitat
It has a distinct south-western bias, but outlying populations, probably a result of recent introduction, occur sporadically. It is heavily synanthropic and typically occurs on dry limestone or loosely mortared walls in towns, villages and farmyards.
It can be found under loose stones, especially capping stones on walls. It may occur in large numbers inside old houses. It also occurs under rocks and stones in railway cuttings and disused quarries where limestone is exposed. It can be much easier to find at night (torch-light surveys) when it is active on the surface. Typically, it is associated with A. vulgare, Porcellio scaber and P. spinicornis.
This summary is based on the detailed account in Gregory (2009).