DEVELOPMENT OF THE SKELETON OF THE TUATARA., 53 
the bones assume a mutual relationship, and is not brought about by ontogenetic 
growth. At Stage R (Pl. III. fig. 12) it will be observed that the anterior edge of the 
palatine (pa.) is transverse and that the vomers (vo.) lie wholly in front of them; 
whereas at Stage S (Pl. IV. fig. 6) the palatine border is angulated and the vomers are 
correspondingly modified. Proof that this change has arisen by backward elongation of 
the vomers, lies in the study of the relationships to the posterior nares, and it therefore 
follows that in the all-characteristic vomero-pterygoid apposition the vomer has played 
apart. Boulenger, one of the advocates of the Rhynchocephalian affinities of the 
Chelonia, has called special attention! to a similar apposition in this order; but con- 
cerning it the process is the reverse of that of the adult Rhynchocephalian, it being 
the vomer which for the most part reaches back to the pterygoid. Judged by the 
foregoing ontogenetic changes undergone by the developing Sphenodon, this feature, 
instead of presenting, as might appear, a difficulty in the way of the acceptation of the 
said affinity, strengthens it, since both bones are in both groups of animals involved 
in the apposition, though in an inverse degree. 
In Ichthyosaurus the vomers extend back divaricatingly behind the nares, and the 
pterygoids, inserting themselves between them, extend forwards between the nares. 
Taking the Stage R of Sphenodon as the starting point, the Ichthyosaur might well 
represent one extreme of modification, the Chelonian, through the adult Sphenodon, 
an opposite one; and in consideration of accepted views of the Chelonian affinities of 
the Plesiosauria °, it is interesting indeed to find that in these they are the yvomers 
which longitudinally extend ¢. 
The interest in the pterygoid of Sphenodon does not stop here, for, on examining its 
basicranial articulation we have found a synovial joint to be present and an indepen- 
dent interarticular cartilage (Pl. 4, figs. 1 & 5, m.p.), which flanks the inner face of the 
pterygoid and has an essential similarity to the meniscus mandibuli. We therefore 
propose to term it the meniscus pterygoideus °. 
Cognate to the study of the pterygoid is the presence of a process of the squamosal 
hitherto unrecognized, which (Pl. IV. fig. 11, sqg.') extends downwards and forwards 
between the quadrate and pterygoid, and, together with a process arising from the 
posterior border of the squamosal (sq.", Pl. IV. fig. 9 and Pl. V. fig. 13), embraces the 
expanded otic head of the quadrate. Both are present in the adult, the hinder process 
being the less conspicuous of the two. 
? Boulenger, G. A.: Brit. Mus. Catal. cit. p. 17. 
* Cf. Baur, G.: Anat. Anz. Bd. x. 1895, p. 458, fig. 1. 
* Of. Andrews, C. W.: Quart. Journ. Geol. Soc. vol. lii. 1896, p. 251, fig. 2. 
‘ Cf. Andrews, C. W.: Geol. Mag. (iv.) vol. iii. 1896, p. 4, and Ann. & Mag. Nat. Hist. (6) vol. xv. 1896, 
p. 345. 
° Parker has undoubtedly seen and figured this in Zootoca (Phil. Trans. 1879, p. 612, & pl. 45, fig. 4), but 
his descriptive paragraph ($7) can hardly be said to adequately describe it. 
