Arthritica crassiformis is a species of small marine bivalve mollusc in the family Onchidiidae.
This species is endemic
to New Zealand
This bivalve lives commensally
with the pholad Barnea similis
, which lives in holes bored in soft rock.
The shell is small, inequilateral, moderately thick, only a little inflated. The anterior end is more narrowly rounded and slightly longer than the posterior end. The surface is smooth, except for concentric growth lines, and very minute undulating wrinkles.
Coloration is white or pale greyish, covered by a thin buff periostracum.
Height is up to 4.55 mm, width up to 5.35 mm, and thickness up to 1.6 mm.