Graph showing the length of time teeth existed in the functioning dentition at different tooth positions, 21 12 Graphic model of the dental history of a young crocodilian, 23 13 Graphic presentation of the dentition of a young crocodilian, derived from the graphic model, 24 14 Stages in the production of the dental lamina and its products, 25 15 Frontal section through the lower jaw of an embryo of Crocodylus porosus, 26 16 (a) A section of the lower jaw of a 7 cm. specimen of Esox lucius. (b) Vorstman's interpretation of the section, 27 17 Early stages in the development of the dentition of Crocodylus embryos, 29 18 Sequence of appearance and arrangement of embryonic dental elements in Crocodylus, 29 19 Charts to show the difference in arrangement of dental elements as conceived by early authors and by the present writer, 32 20 Stages in the formation of the functional dentition, 34 21 Diagram to show the number of teeth which must be discarded before a complete functional tooth row can be laid down, 35 wear, injury, etc. Parrington (1936), Romer and Price (1940) and Edmund (1960) showed that this replacement frequently occurs in regular waves passing along the jaw, usually from back to front, affecting alternately numbered teeth. Thus, in any particular area of the jaw, the odd-numbered teeth are replaced by one wave, followed by replacement of the evennumbered teeth by a succeeding wave. Deeply entrenched in the literature of tooth replacement is the Distichy Theory of Bolk (1912 et al.). This states that there are separate loci for the development of odd-and even-numbered teeth. (1) All of the teeth produced at one set of loci comprise one odontostichos. (2) A complete dentition, according to Bolk, is made up of the alternating elements of two odontostichi. (3) This gives rise to simple alternate replacement, i.e., all of the teeth of one odontostichos are replaced at one time. followed by simultaneous replacement of all of the teeth of the other odontostichos. Bolk described crocodilian embryos in which there appeared to be simple, alternate replacement. The same observation was noted earlier by Rose (1893) and by others. Wocrdeman (1919 and 1921) studied crocodilian embryos by serial sectioning, and showed that Bolk's concept of two sites for the production of replacement teeth was in error. Vorstman (1922) reviewed the work of Bolk and Woerdeman and, while not working specifically with crocodilians, derived some interesting generalities. Unfortunately, the terminology used by Bolk, Woerdeman and Vorstman is confusing and occasionally misleading. Edmund (1960) explained and redefined the essential terminology. The dentitions of a number of post-embryonic crocodilians were analysed by Edmund (1960) who remarked that the regular pattern of replacement seen in other archosaurs was often lacking. Some specimens had replacement occurring in waves progressing from back to front. In others the reverse was noted. Some showed no recognizable pattern. Since this differed so markedly from the usual reptilian pattern, ...
