Shell large, white, hinge line set very obliquely. Anterior end narrowly rounded, posterior end widely expanded. Beaks set very near anterior end. External sculpture consisting of irregular growth lines and obsolete radials. Internally there are two strong ridges separated by grooves and a number of subsidiary ridges extending from the posterior end towards the beaks but not reaching them. The outlines of these ridges and grooves show up externally especially near the posterior margin. Hinge plate narrow with 3 anterior and 5 posterior blunt teeth set partly below a long narrow, vertically striated ligamental area. Ligamental area broken in middle by a narrowly triangular resilium pit. Anterior and postero-dorsal margins obliquely crenulated. Height, 6,7 mm.; length. 4,8mm.; inflation (single valve), 1,4 mm.
L. benthicola can be separated from the known New Zealand species by the large size, oblique hinge line and the internal ridges.