Comparison of dorsal shell ribs. Shell heights both 11.2 mm. Where ribs from both sides meet, they are:

on T. monacha, (1) unaligned, (2) often of different width or (3) joined at an angle.

On T. arctica, ribs are perfectly aligned and their junction point is imperceptible, except where deflected by an interposed subsidiary rib (4).

Albums of OTHER SPECIES:
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