G4 



PEOF. G. B. JIOWES AND ME. 11. II. SWINNERTOX ON THE 



(PI. II. fig. 15), the outer tooth of each set being somewliat tlio larger of the throe. At 

 Stage S (fig. 14) all are seen to be free, and the outer still large. Beyond the mere 

 inference of fusion, Dendy does not state the method by which he presumes these 

 ]iointed incisors to be converted into the tusks of the adult. As a rule each of the 

 upper tusks, when fully formed, is doubly pointed (or, as remarked by Gunther, 

 " notched") {cf. text-fig. 15), there being a smaller internal cone, and a larger external 

 one somewhat backwardly curved (text-fig. 16). In the lower jaw there arc three cones 

 (text-fig. 16), a larger external and two smaller internal, the three teeth which are 



Fig. 17. 



Fiff. 15. 



Fig. 10. 



Fig. 18. 



Illustrations of the dentition of Sphenodon. 



Fig. 15. Incisors, front aspect, old individual. Fig. 16. The same, younger individual, little worn. Both 

 nat. size. Fig. 17. Enlarged sketch of upper incisors of fig. 16, x 6. Fig. 18. Complete adult dentition : 

 Bat. size, i, incisors; t.a., cheek-teeth, alternating series; t.u, cheek-teeth, structurally uniform series. 



present having united with the mandible. That portion of the dentary element which 

 bears them becomes, with them, clothed in an enamel-like substance, which justifies 

 Giinther's remark (67, p. 602) that the alveolar edge of the mandible is polished. 

 Tomes ^ regards this substance as true bone. 



In the embryonic upper jaw the middle incisor present early ceases to elongate 

 [cf. PL II. fig. 15, t.m.) and at Stage T it becomes loose. The reason of this is 

 evident from the fact that longitudinal section (PI. II. fig. 16) shows it to be confluent 



' Tomes, C. : Dental Anatomy, ed. 4, 1894, p. 253. 



