DEVELOPMENT OF THE SKELETON OF THE TUATAKA. 63 



fig. 18, in all but old individuals. Bayer, finding the prseaxial moiety of the " astra- 

 galus " cartilaginous, regarded it as the tibiale, and the whole " astragalus " as con- 

 sisting of the centrale l and the intermedium, the " calcaneum " being to him the 

 fibulare (op. dt. p. 243). Perrin, going further, names the " astragalus " (op. cit. 

 p. 44) the " tibio-centro-intermediare." 



Development at Stage Q shows (PI. VI. fig. 16) that at the chondrification period the 

 " astragalus " and " calcaneum " are independently chondrified, and a feeble ypsiloid 

 differentiation (accurately indicated in the fig.) may be observed at what would be 

 the apposition lines of the tibiale (.), centrale (tc.), and intermediare (?'.), were they 

 distinct. In an earlier embryo of the same stage (fig. 15) we found the inter- 

 mediare -(i.) and fibulare (/.) chondrified and separated by a foramen apparently 

 homologous with that known as the f. arteria perforans mesopodii (/$.), the 

 centrale (tc.) and tibiale (t.) being procartilaginous, although in close apposition with 

 the intermediare and with each other. Bayer and Perrin had correctly surmised! 



7. THE DENTITION. 



The detailed study of the tooth-genesis does not fall within the scope of our work, 

 but we have one or two interesting anatomical facts to record. 



Firstly, as to the vomerine teeth. No trace of them in a calcified form is forth- 

 coming up to Stage T, the oldest we have examined. We can add nothing to what is 

 recorded in the latest account given by Siebenrock 2 . It maybe that we have not 

 happened to deal with embryos in which they were developing, or, perhaps, that these 

 clearly vestigial organs attain their full development late. 



Baur would seem to be wrong in the surmise (96 b . p. 437) that they exist in 

 young specimens and disappear in old age 3 . 



The Incisors. These have been correctly shown by Dendy (99 a . p. 77) to be repre- 

 sented in the young stage by three pairs of " distinct pointed conical teeth " above and 

 below, and not by two as believed by Giinther (67. p. 602) and Baur (p. 436). It is 

 worth remarking, however, that Knox had already recorded the supposed existence of 

 three pairs in the upper jaw (69. p. 18), and Newman in the lower (77. p. 234), the 

 former stating that they were confluent at their bases. This is the case at our Stage T 



1 Baur's passing remark (Zool. Auz. Bd. ix. p. 189) that he found this represented by a piece of cartilage 

 which was " eingekeilt " is suggestive of begging the question. 



2 Siebenrock, F. : Transl. cit. p. 309. 



3 Further search is necessary to ascertain how far the vomerine teeth are or are not represented during 

 ontogeny, and a former pupil of mine, Mr. H. Spencer Harrison, has undertaken to immediately work out this and 

 other problems connected with the dentition of Sphenodon as the subject of a special communication to appear 

 in the Qu. Journ. Microscp. Science. G. B. H. 



