66 PROF, G. B. HOWES AND MR. H. H. SWINNERTON ON THE 
upper incisors 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 
19 mm. example we find a similar tooth (PI. II. fig. 16, ¢d.) but behind the middle 
tooth of the three that are present (¢.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 
the tooth is never functional receives support from what we have observed 2. 
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 (¢.w.), we find these to mostly arise lineally. Pl. II. 
fig. 17, in its outlined portion, is a reconstructional drawing from horizontal microscopic 
sections at this period. ‘The teeth drawn in outline (¢,/) are fused to the maxilla and 
palatine, as indicated. Those drawn in colour, together with the shaded portion of the 
drawing, 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 
* acrodont 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 “acrodont,”’ aud he further suggests that 
the glistening 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 their being in the adult non-acrodont, we have toe 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 én sitw; 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 Pl. I. fig. 18 shows in the case of a mandibular tooth that the 
truly acrodont 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. 
1 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. 
