DEVELOPMENT OF THE SKELETON OF THE TUATARA. 65 
only externally with the premaxilla (p.a.) and not by its whole base, as is the case with 
the other teeth. We suspect that this tooth is early shed, but unfortunately our material 
does not embrace a specimen proving the point. 
A possible source of error must not be here overlooked, in the fact that in the adult 
there may be present a small pointed cusp (text-fig. 16,*) between the two upper 
incisors, which at first suggests a persistent middle tooth. When, however, a tooth 
thus constituted is examined under a lens, it is found that the minute cusp has a 
counterpart (text-fig. 17) on the inner face of the inner tooth, and that, like this, it 
is but the product of its base. And since the teeth, when first formed, are simple 
cones, these lesser cusps must be due to secondary complication. 
The palatal and maxillary teeth vary very greatly in appearance at different stages in 
development and periods of life. One conspicuous feature which has not been recorded 
is that at Stage S the maxillary teeth number 11 and the palatal 4-5 (cf Pl. IV. 
figs. 4-6). These first-formed cheek-teeth usually alternate, a larger and a smaller 
occurring in succession, and not infrequently they may be arranged in recurring sets of 
three, a larger and two smaller to each. At Stage T the same numerical condition 
obtains (allowing for individual variation, most marked for the palatine teeth), but 
under the growth-processes at work the larger teeth have now increased in size. 
From this period onwards the numerical increase in the teeth as a whole is due 
to superaddition from behind forwards, and most of the teeth thus formed (t.w., text- 
fig. 18) are uniform in size. Numerically they average about 6 on either side, and 
for the palate about 4-5, giving a total number of about 16-17 maxillary teeth and 
9-11 palatal !. 
Passing to the mandibular series, we note an essential similarity to the maxillary, in 
the arrangement of the teeth first formed. At Stage S (Pl. 1V. fig. 4) there are 11 
alternating teeth as in the maxilla, but no others. On the assumption of Stage T 
the teeth of the alternating series, unlike those of the upper jaw, apparently cease to 
grow. Wear and tear supervening, and the development of the structurally uniform 
teeth continuing apace, these (t.w., text-fig. 18) become predominant; and hence the 
condition herein delineated, which is of short duration, as the teeth of the alternating 
series (¢.@.), now insignificant, become rapidly worn down. 
Baur has described (95°. p. 456) a Sphenodon skull, 25 mim. in length, having two 
* The number of teeth recorded by different observers ranges for the maxillary from 6 (Knox) to 17 (Colenso), 
and for the palatal from 5 (Knox) to 13 (Newman); for the mandibular from 13 (Colenso) to 19 (Giinther). 
In the absence of detailed description and knowledge of degree of wear and tear, it is not possible to make 
use of these figures. 
> This alternation is a striking feature. Among the Monitors it is often the case that in the fresh state the 
cheek-teeth are similarly, though more markedly, alternate, the elongated teeth being fixed to the Jaws alternately 
with small ones which are freely displaceable at their bases. Can it be that there is a common determining 
cause at work here? 
VOL. XVI.—PART I. No. 9.—February, 1901. K 
