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

Description.—Simple, shallowly vertical U-shaped tunnels, with apertures at low angles to shell surface (Fig. 5.4). Tunnels are straight to moderately curved, with uniform diameters, infrequently possess swollen or tapering chambers (when a U-shape was not fully developed), oriented upright, inclined or parallel  to the substrate surface; clavate chambers only slightly larger than neck and aperture. Tunnels have smooth walls, are circular in cross section (mean diameter of 0.2 mm) and are variable in length; branching/forking is infrequent (7.2%) and tunnels possess no indicators of a significant lining. Tunnel entrances are circular to elliptical but are not typically preserved. Tunnels are located within primary and secondary shell substrate; oriented parallel, perpendicular or at various angles to host growth lines with no specific preference.