Thanks to the expert whose voluntary task it is to verify any slug records input to the local biodiversity records database, I now know that what I thought was a Leopard slug (Limax maximus) is actually a Green cellar slug (Limacus maculatus). The expert kindly explained: ‘You are far from the first person to have made this misidentification. Limax maximus is brown, rather rougher and dryer in texture than either of the Limacus species,[and] is usually solitary.’ Always learning!