f,(i PKOF. G. H. HOWES AND MR. 11. H. SWlNKEJiTON ON THE 



upper iucisors and an eminence which he surmised suggested a third, and he remarked » 

 that each of the inner incisors on either side had a very small successional tooth, which 

 he believed was never further developed. On examination of a series of sections of a 

 ]9 mm. example we find a similar tooth (PL H. fig. 16, td.) but behind the middle 

 tooth of the three that are present (t.m.). Baur's specimen appears to have suffered the 

 loss of the middle incisor, and to justify our conclusion that this is shed. And while 

 we are not sure whether our successional incisor is the same as his, or whether he and 

 we were dealing with members of a series which may be formed, his conclusion that 

 tlie tooth is never functional receives support from what we have observed -. 



In describing the cheek-teeth, Baur attributed to the fifth maxillary and the first 

 mandibular a successional tooth. This we have not been able to confirm. Passing to 

 the structurally uniform series (t.u.), we find these to mostly arise lineally. PI. II. 

 fio-. 17, in its outlined portion, is a reconstructional drawing from horizontal microscopic 

 sections at this period. The teeth drawn in outline (t.f.) are fused to the maxilla and 

 palatine, as indicated. Those drawn in colour, together with the shaded portion of the 

 drawinf^ are from individual sections, the teeth which appear to represent the structurally 

 uniform series (coloured yellow) being delineated in the position in which they arise. 



The teeth of the adult Sphenodon have been defined by Giinther (67. p. 601) as 

 '• aerodont in the strictest meaning of the term." Boulenger (Brit. Mus. Cat. cit. p. 1) 

 regards them as " not implanted in alveoli " ; while Tomes, ignoring the incisors, has 

 written of the post-incisors that they are " aerodont," and he further suggests that 

 the o-listenino- investment of the alveolar edges, which Giinther originally described and 



showed to " perform the functions of teeth when these are ground down 



in advanced age," is true bone, Baur refers (p. 437) to " alveoli " in his 25 mm. 

 specimens, but gives insufficient details. 



If we may infer from the foregoing that Tomes, ignoring the incisors, was suspicious 

 of then- being in the adult non-acrodont, we have to confirm his doubts. Thanks to 

 Mr. M. F. Woodward, we have been enabled to examine a series of microscopic sections 

 of both upper and lower incisors of the adult in situ ; and concerning the upper 

 there is evidence of surrounding bone — not as the result of a truly thecodont condition, 

 but of an apparent secondary overgrowth subsequent to the union between tooth and 

 jaw. Examination of PI. II. fig. 18 shows in the case of a mandibular tooth that the 

 truly aerodont condition is assumed at Stage S ; and at Stage T the incisors show in 

 our sections a condition intermediate between this and that above described. Further 

 investigation of this topic is beyond the limits of our pledge to Professor Dendy, 

 but, pending detailed study of the tooth-genesis, we would distinguish the condition 

 occurring in at least the incisors as hyperacrodont. 



' As usual, he writes (line 8) " innere " for " aussere." 



" We have no material to show whether the embryo teeth which we herein describe do or do not become 

 later replaced. 



