Letronne is the lava-flooded remnant of a lunar crater. The northern part of the rim is completely missing, and opens into the Oceanus Procellarum, forming a bay along the southwestern shore. The formation is located to the northwest of the large crater Gassendi.To the west-southwest is the flooded crater Billy, and north-northwest lies the smaller Flamsteed. The surviving rim is now little more than a semi-circular series of ridges. (wikipedia entry)