Diagnosis.—Cylindrical tunnels, straight to moderately curved, circular in cross section with very small diameters (~0.2 mm), variable in length; branching/forking infrequently; walls smooth. Fully developed tunnels vertically U-shaped, entering and exiting at very low angles to the shell surface; tunnel entrances and exits vary from circular to elliptical. Tunnels that are not fully  U-shaped may terminate with a tapering or swollen chamber or maintain a uniform diameter along the length of the tunnel. All borings are located within the upper and middle shell substrate of the host.

Category of architectural design: 2.61. U-shaped borings.

Microboring; substrate calcareous; tracemaker invertebrate