Adult T. monacha is rarely, if ever, as small as this 8.5 mm; usually 10 mm and over.

1: pitch-brown/black dorsal marks on fully extended mantle of T. arctica are often confused with similar marks on shell of T. monacha. Mantle translucent, sometimes difficult to tell if marks on or below mantle unless touched to make it expose shell.

2: dorsal surface of T. arctica foot lacks lines (clearly defined lines on T. monacha).

Shell height (longest dimension) 8.5 mm. Menai Strait, Wales. March 2011.