DEVELOPMENT OF THE SKELETON OF THE TUATAEA. 53 



the bones assume a mutual relationship, and is not brought about by ontogenetic 

 growth. At Stage E (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 (PI. 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 

 a part. Boulenger, one of the advocates of the Rhynchocephalian aflanities of the 

 Chelonia, has called special attention ^ to a similar aj^position 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 Avay 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 divaricatingiy 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 vomers 

 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 (PI. 4, figs. 1 & 3, m.p.), which flanks the inner face of the 

 pterygoid and has an essential similarity to the meniscu.s 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, sq.') 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. IS), 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. 



' Cf. Andrews, C. "W. : Quart. Journ. Geol. Soe. 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 Zootooa (Phil. Trans. 1879, p. 612, & pi. 45. fig. 4), but 

 his descriptive paragraph (§ 7) can hardly be said to adequately describe it. 



