Tooth development is traced in dentitions from 169 embryonic Lamna nasus (96–670 mm fork length [FL]), and 8 juvenile to adult L. nasus (67–300 cm FL). The teeth of these dentitions vary significantly in the attitudes and shapes of their crowns, and the forms of their roots. At some stages of their ontogeny they bear characters of carcharhiniform and alopiid shark teeth, and they may also bear ornamentation on their crowns. In early embryos, the teeth are conical with funnel-shaped roots and resemble skate dermal thorns, Raja spp. Tooth shape changes abruptly at about 350 mm FL to a late embryonic dentition, with very elongate lower anterior teeth, as in Pseudocarcharias. Adult teeth also exhibit significant morphological variation, and exhibit some characters that occur also in primitive neoselachians, such as Synechodus and Paraorthacodus. The importance of these tooth ontogenetic and morphological variations for the study of fossil shark teeth is presented, and a generic diagnosis is offered for juvenile and adult Lamna teeth.