The tooth that replaces the primary first molar located in the permanent dentition of the mandible between the canine and the second premolar tooth. The crown is roughly circular in outline, although slightly flattened mesio-lingually. It has one large buccal cusp and one much smaller lingual cusp whose apex is at a much lower level than the buccal cusp. The cusps are invariably connected by a central ridge running bucco-lingually, on either side of which is a pit (fossa). The distal pit is normally larger than the mesial pit. Marginal ridges bound both pits and connect with the two cusps. Frequently a groove runs from the mesial pit over the mesial marginal ridge onto the mesio-lingual surface. The crown is inclined lingually, so that the apex of the buccal cusp lies directly over the vertical axis of the root. The mesial and distal surfaces are convex and slope towards the narrower lingual surface of the root. There is normally only one root with vertical grooves, on the flattened mesial and distal surfaces of which the mesial is more marked. There is normally a single root with a pulp chamber having one pulp horn extending towards the apex of the buccal cusp and a single pulp canal, although this may be subject to considerable variation. Calcification of the tooth begins at about 1½–2 years after birth and the crown is normally complete by 5–6 years of age. The tooth erupts at about 10–12 years and the calcification of the root is complete at about 12–13 years.