DEVELOPMENT OF THE SKELETON OE THE TUATAEA. 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. PI. 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.u., 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 K 



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 IV. 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.u., text-fig. 18) become predominant; and hence the 

 condition herein delineated, which is of short duration, as the teeth of the alternating- 

 series (t.a.), now insignificant, become rapidly worn down. 



Baur has described (96^. p. 436) a Sphenodon skull, 25 mm. 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 thejaws alternately 

 with small ones which are freely displaceable at their bases. Can it be that t-here is a common determining 

 cause at work here ? 



VOL. XVI. — PART I. No. 9. — February, 1901. k 



