Final juvenile stage of T. arctica with ribs but no pigment. Shell is identical to T. monacha at this stage except that the ribs from left and right are perfectly aligned and meet dorsally without anything to mark the junction point. On T. monacha the ribs rarely all meet without irregularity or misalignment of some ribs. H. 11 mm. Bretagne, France, August 2021, © M. Adam.