Among the creatures I photographed and collected in my sister's backyard in Montreal when I was visiting them earlier this month was this isopod. I have identified it as Porcellio spinicornis. Like the snails from the same location, these isopods are also introductions from Europe.
Before we any further, here is a disclaimer: I am a novice at identifying isopods, so I may be totally wrong. I have relied on the shape of the frontal lobe of the head (red arrows in the pictures below) to distinguish P. spinicornis from P. scaber. Both of those species have been recorded from Quebec, Canada1. The frontal lobe of the head is supposed to be rounded in P. spinicornis and triangular in P. scaber.
A: Porcellio spinicornis from Montreal; B: Porcellio scaber from Maryland. Both specimens were photographed alive. Arrows point at the frontal lobe of the head.
My authority on this is Edney2, from which source I adapted the following drawing. Compare it with the photos above.
1Joan Jass & Barbara Klausmeier. 2001. Terrestrial Isopod (Crustacea: Isopoda) Atlas for Canada, Alaska, and the Contiguous United States. Milwaukee Public Museum Contributions in Biology and Geology #95: 1-105.
2E.B. Edney. 1953. The woodlice of Great Britain and Ireland. Proc. Linn. Soc. London, 164:49.