Lamarck is a crater in the southwestern part of the Moon. The northern portion of the crater is overlain by the walled plain Darwin. To the southeast is Byrgius. The outer wall of this formation has been heavily damaged by subsequent impacts, leaving a disintegrated rim that forms a low, irregular ridge in the surface. Parts of the crater are also overlain by ejecta from the Mare Orientale basin to the east. The most notable feature in the interior is the tiny, bowl-shaped impact crater Lamarck B. The remainder of the floor forms a rolling, uneven plain.