DEVELOPMENT OF THE SKELETON OF THE TUATARA. 63 
fig. 18, in all but old individuals. Bayer, finding the preaxial moiety of the “astra- 
galus”” cartilaginous, regarded it as the tibiale, and the whole “astragalus” as con- 
sisting of the centrale! and the intermedium, the “calcaneum” being to him the 
fibulare (op. cit. 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 (#.) and fibulare (f.) chondrified and separated by a foramen apparently 
homologous with that known as the f. arteria perforans mesopodii (f%.), the 
centrale (¢c.) and tibiale (¢.) 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?. It may be 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°. p, 437) that they exist in 
young specimens and disappear in old age °. 
The Incisors.—These have been correctly shown by Dendy (99*. 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 
* Baur’s passing remark (Zool. Anz. 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. 
