Porcellio scaber vs dilatatus
Image showing Porcellio scaber (above) and Porcellio dilatatus (below) for habitus comparison. As the name suggests dilatatus is "wider". Also the colour is somewhat different (but P. scaber is very variable in colour!) and there are some less obvious morphological differences (telson and head shaped differently). The two are not always easy to tell apart from photo or in the field.
The Rough Woodlouse (Porcellio scaber) is very, very common and widely distributed in Europe as well as introduced to many other parts of the world. Porcellio dilatatus is also wide spread and introduced to North America, but it is rather scarce. In the Netherlands its relict habitats (mainly around very old farms or other old buildings or ruins or in caves/mines) are rapidly disappearing. Three of the five locations where I've ever found it in the Netherlands are now gone. The location of this photo is one of them (old ruin of farm replaced by modern building).
I've identified this image as P. dilatatus as it is the rarer species of the two
''Porcellio dilatatus'' is a species of woodlouse in the genus ''Porcellio'' belonging to the family Porcellionidae. This species is widespread in Europe, and has also been introduced to North America from Western Europe. They are 15 millimetres long, are brown coloured and striped. They can be found feeding on alder leaves, but mostly feeds on organic food substrates, such as lettuce in the wild. It also feeds on inorganic metal salts.