Acaenoplax is an extinct worm-shaped mollusc known from the Coalbrookdale Formation of Herefordshire, England.

[1][2] Some of its characters are reminiscent of the polychaete worms, and the character combinations do not place it obviously in the stem of any modern mollusc group,[3] but although it was originally interpreted as a polychaete,[1] this position is untenable for a number of reasons.

[4] The organism resembles a bristled worm, but bears a number of shells on its upper surface.

[1] There are eighteen rows of spines projecting from ridges in the body surface, which encircle the body except for its bottom surface,[1] which presumably bore a molluscan foot.

[2] Heloplax, Enetoplax and Arctoplax are genera of shell that are closely related to Acaenoplax, but whose soft tissue is not preserved.