Animai Dentition 225 
= use longer than possible, a number of successional teeth in 
various stages of development grow up behind each fang, and 
remain lying flat on the palate till their turn comes for 
functional employment. Such is the highest development of the 
poison-apparatus among venomous serpents, but there are many 
kinds in which this culminating stage has not been attained. 
In these the venom-teeth are not greatly larger than the 
rest, are grooved instead of tubular, and remain permanently 
erect instead of lying flat on the palate when not in use. 
, A dental development quite unlike that of any other living 
reptile occurs in the tuatera, a creature inhabiting two small islands off New Zealand 
and commonly regarded as a lizard, although its organisation is widely different from that 
of the reptiles properly so called. The dentition of the tuatera (Figs. 9 and 10) comprises 
a pair of chisel-like teeth in the front of each jaw, a double row of closely-packed small 
teeth, separated by a deep groove, on the sides of the upper jaw, and a single row of 
similar teeth on the summit of the sides of the lower jaw. When in use, the lateral 
series of lower teeth bite into the groove between the two corresponding rows of upper 
teeth, and in old individuals, when the teeth become much worn away, the edge of 
the lower jaw itself becomes highly polished and acts as a cutting-instrument. What 
may be the precise use of this unique 
type of dentition does not appear to have 
been hitherto explained. In captivity, 
tuateras are reported to devour meal- 
worms and other insects with avidity, 
but if is considered probable that in a 
state of nature vegetable substances form 
a considerable proportion of their diet. 
The tuatera is, however, not only 
interesting on account of its remarkable 
dentition and other structural peculiarities, 
but likewise from the circumstance that 
if is the solitary survivor of a once 
numerous and widely-spread group of 
reptiles, some of the members of which ee Aes 
display an ultra development of the same Be Te ee oe ee LORE DR 
dental type. The best known of these 5 
is the pavement-toothed tuatera (Hyper- 
odapedon), which attamed a length of 
from five to six feet, and has left its 
remains in the older Secondary (Triassic) 
rocks of this country and India. In 
place of the double row of lateral upper 
teeth of the living tuatera, the pavement- 
toothed species (Fig. 11) has a number 
of such rows, so that nearly the whole 
palate is covered with a mosaic of blunt 
pyramidal teeth, between the outer and 
second rows of which worked the single 
row of lateral lower teeth and the sharp 
upper edge of the jaw itself. It is, of aa Ec EAT aa tee eae Jj | 
course, quite impossible to determine the "Reptile. / 
Fig. 10 Side View of Dentition 
of Tuatera. 
