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In Hemiphractus fang–like teeth are ankylosed to the premaxilla, maxilla and prevomer, and bony odontoids are found on the dentary, angular and palatine bones. The odontoids are small, but a larger pair at the front of the lower jaw project upwards and backwards into the mouth and fit into a diastema between the anterior premaxillary teeth when the mouth is closed. The teeth are unipartite and monocuspid, and each consists of a strongly recurved and elongated cone of orthodentine, capped at the tip by a thin layer of enamel. The inner circumpulpal layer of the dentine is tubular, but no tubules are present in the outer pallial layer. During tooth development, dentine is formed before the enamel matrix is produced, and the tooth germs lie horizontally beneath the ventral surface of each dentigerous bone. On eruption, the tooth germs migrate horizontally and become ankylosed to the outer edge of the jaw bone by a layer of cellular cementum. During tooth replacement, the vast majority of the dentine of each tooth, and the cementum at the tooth base, are resorbed by osteoclasts. It is not clear whether the tips of the teeth are shed or not.

A study of the tissues of the teeth and jaws in piranhas, using the scanning electron microscope and various techniques of light microscopy, revealed many dental adaptations related to the specialized feeding habits of these carnivorous fishes. The dentition is primarily sectorial, although some anterior teeth may be used in grasping. The scissor-like rows of teeth are maintained by the specialized pattern of tooth replacement. The bones of the jaws and the tooth attachment support the teeth very firmly. In its structural organization, the enameloid covering the teeth closely resembles that on the sectorial teeth of sharks and is adapted to the probable stress patterns set up in biting.

Development of the lower dentition from stage 19 (day 26) embryos to 11 days after hatching (day 76) was studied in a close series of accurately aged (and staged) specimens of Alligator mississippiensis using macroscopy, light microscopy, scanning electron microscopy, radiography and detailed reconstructions. This study complements our earlier investigations of lower dentition development in alligator embryos from initiation to stage 18, day 26 (Westergaard & Ferguson, (1986) and so provides the first careful documentation of dentition development in any non-mammalian tetrapod from the first initiated tooth to the first erupted and functional dentition. In this way, it is possible to test previous models of dental development, e.g. Woerdeman (1919) suggested that four Odontostichi would be resorbed from initiation to eruption, and Edmund's (1962) Zahnreihe theory predicts that 84 embryonic teeth would be resorbed in this time period. Both are wrong: approximately 19 early teeth (resorptive group) are resorbed or shed without becoming functional; seven teeth (transitional group) function for a short period (less than two weeks) or are sometimes resorbed or shed without becoming functional and 36 teeth (functional group), initiated during embryonic life, function for longer periods. All teeth produce dentine, but a differentiated enamel organ is absent in teeth 1–8, 9b, 10, 12 and 14: these teeth never produce enamel. Teeth 9a and 11 have a poorly differentiated enamel organ and form only a thin layer of enamel; the remaining teeth develop a typical mammalian enamel organ and produce enamel. As several resorptive teeth produce enamel, there is no simple relationship between dental competence and function. Tooth families are rather arbitrarily defined before separation by connective tissue, thus supporting the Tooth Position theory (Westergaard, 1980, 1983). Differential jaw growth enables the establishment of up to five intervening tooth families between families of the first tooth row. Growth is most pronounced in the middle of each jaw half. Except for teeth 1 and 3, the teeth from odd- and even-numbered families can be fitted into fairly smooth initiation curves. The most active initiation phase occurs between embryonic days 21 and 33. By hatching, families founded by teeth 1 to 5 (3, 6, 12, 2 and 4) have developed four generations of teeth, families 17, 19 and 20, two generations and all other families, three generations of teeth. The interval of time between initiation of successive teeth from the same tooth family ranges from 6–26 days. One clutch of embryos showed a disappearance of tooth family 6 from one jaw side and another clutch exhibited an extra tooth family 0: both these clutch-related anomalies were associated with differences in jaw growth. The embryonic dental system seems to be evolutionarily very plastic, changes of jaw growth enabling the creation or removal of tooth positions anywhere in the odontogenic area. A new progress zone model of dentition initiation is proposed based primarily on a positional information system in the jaw epithelium.