Animal Dentition 
Such a single replacement must, however, almost 
certainly have occurred in a very remarkable herbivorous 
member of the same group scientifically known as Tr- y, 
tylodon. In this reptile the molars (Fig. 14) have broad  / 
longitudinally-ridged crowns, and are inserted by double 
or treble roots, and the front of the upper jaw carries 
a pair of large tusks, _but whether there were small ee Peo 
incisors between them is not known. So mammal-like, Transverse Section of a Tooth 
indeed, is the whole dentition that the creature was for Co eae eae: 
a lone time regarded as a mammal, and it was only when certain characteristically 
reptilian bones were ascertained to be present in the skull that it was relegated to a 
lower grade in the animal kingdom. In reality it probably forms a connecting link 
between mammals and reptiles, with a certain claim to be included in both classes. 
Quite unique in the class are the curious single-tusked extinct reptiles known as 
dicynodons, which were near allies of the carnivorous mammal-like forms. In these, as 
shown in Fig. 15, the upper jaws were furnished with a single pair of enormous 
downwardly-directed tusks, but with this exception both jaws were destitute of teeth 
and probably encased in horn, from which we may infer that these reptiles were 
probably herbivorous. In the same (Triassic) strata occur remains of other reptiles 
similar in all respects to the dicynodons, but without upper tusks. They have been 
regarded as indicating a distinct genus, but it is far from improbable that they are 
female dicynodons. If so, the tusks may have been offensive weapons used in combat 
between rival males. Some of the dicynodons were huge creatures, with skulls as 
large as those of crocodiles, although shorter and higher. 
Many other remarkable types of dentition are presented by the mammatl-like 
reptiles and their kindred, but if I were to allude to these this article would be spun 
out to an inordinate length. From the lower types of these reptiles there are some 
indications of a transition towards the still earlier and more primitive creatures known 
as Labyrinthodonts, or primeval salamanders, some of which had a skull nearly a yard 
in length. The most striking peculiarity in the dentition of the typical members of 
this group is to be found in the complicated internal structure of their teeth, from 
which they derive their technical title. As shown in Fig. 16, a section of a tooth 
displays a number of irregular canals radiating in a “dendritic” manner from a 
central pulp-cavity. A very similar type of tooth-structure occurs in certain extinct 
fishes, but the object of such a complicated arrangement is quite unknown. 
This very sketchy account of reptilian dentition must be brought to a close by a 
brief reference to the teeth of the largest of all known veptiles, the extinct dinosaurs, 
some of which walked on all fours and attained a length of fully sixty feet, while 
others stalked about on them hind 
legs like gigantic kangaroos, towering 
some twenty feet above the ground. 
Some, by no means the largest of 
these creatures, such as the megalo- 
saur, were evidently carnivorous, as 
is proved by their powerful dental 
armature of compressed and recurved 
teeth with sharp and serrated edges 
(Fig. 17). Others again, as typified 
by the well-known iguanodon of the 
Weald of Kent and Sussex, were as ; 
clearly herbivorous, the crowns of the Fig. 17, Teeth of the Megalosaur. 
