The South American lungfish Lepidosiren has toothplates bearing an extremely hard version of dentine: petrodentine. The hardness of this tissue, and its associated ordinary dentine, was compared with that of the enamel, dentine and cement of mammalian teeth, and also with that of other mammalian bony tissues. The hardnesses of petrodentine and dentine of Lepidosiren were found to be similar to those of enamel and dentine in other, mammalian, teeth. Furthermore, the anatomical arrangement of the Lepidosiren tissues was similar to that found in the incisors of rodents, and they presumably function in the same way to keep a sharp chisel edge at the tip of the tooth. Comparison of fracture surfaces of Lepidosiren petrodentine and that of rat incisor showed, however, that petrodentine does not have the refined, crack-stopping structure found in rat incisor enamel.