In the Reptilia various arrangements of the teeth are found. In the Chelonia (turtles) there are no teeth, although the ecto dermal ingrowth (dental band) from which they are developed in other animals is present in the embryo. The place of the teeth in these reptiles is taken by horny jaw-cases.
In the Ophidia the non-poisonous snakes have two rows of teeth in the upper jaw, one on the maxillae and another on the palatine and pterygoid bones, while in the lower jaw there is only one row. These teeth are sharp pegs anchylosed to the bones and so strongly recurved that one of these snakes would be unable, even if it wished to do so, to let any prey which had once entered its mouth escape. The poisonous snakes have a special poison fang in the maxilla of each side ; these have a deep groove or canal running down them which transmits the poison from the poison gland. In the colubrine snakes, such as the cobra, the poison fang is always erect, but in the viperine, such as the adder and rattlesnake, there is a mechanism by which the tooth is only erected when the jaws are opened for striking. At other times the teeth lie flat in the roof of the mouth.
In the lizards or Lacertilia the teeth usually consist of a series of pegs in the upper and lower jaw, each resembling the one in front of it ; sometimes, as in the chameleon, they are anchylosed by their bases to the bone, but at others, as in the iguana, they are fused by their sides to a ridge of bone which forms a low wall on their lateral surface. In the former case the dentition is spoken of as "acrodont," in the latter as "pleurodont." In the Crocodilia the teeth are fitted into definite sockets as in mammals and are not anchylosed with the jaws. This arrangement is spoken of as "thecodont." Existing birds are toothless, but palaeontology shows that they originally had teeth of a reptilian character.
In all these lower vertebrates, then, the teeth are similar or nearly similar in character; at least they are not divided into definite incisor, canine, premolar and molar regions. Their denti• tion is therefore known as "homodont." Another characteristic is that in almost all of them there is an arrangement for a continuous succession of teeth, so that when one is lost another from behind takes its place, and to this arrangement the term "polyphyodont" is applied.
Mammalia.—In the Mammalia the different groups of teeth (incisor, canine, etc.) already noticed in man are found, and these animals are characterized, with some exceptions, by having a "heterodont" as opposed to a homodont dentition. In the mam mals too the polyphyodont or continuous succession of teeth is reduced to a "diphyodont" dentition, which means that there is only one relay of teeth to replace the first set. In the marsupials the reduction of the succession is carried still further, for only one premolar in each segment of the jaw is replaced, while in the toothed whales there is no succession at all. When one set has to do duty throughout life the dentition is called "monophyodont." It is uncertain how the complex back teeth of mammals with their numerous cusps were derived from the simple conical teeth which are generally assumed to have been the primitive arrange ment. The "tritubercular" theory, which is most favoured, is largely based on the researches of E. D. Cope and H. F. Osborn, two American palaeontologists. According to this theory a simple peg-like ("haplodont") tooth develops two additional smaller pegs or cones, one in front and one behind the original main cone. This is known as the triconodont stage, and it is found in some of the oldest extinct mammals. As a later adaptation the two small cones become external to the original cone in the upper jaw and internal in the lower. The surface of the tooth has now a tri angular shape with a cone at each angle, and this is the "tri tubercular tooth" which is very common among the ancestral mammals. Other cusps may be developed later, and so the quadricuspid and quinquecuspid molar teeth of man and other mammals are accounted for.
Incisor teeth are those which in the upper jaw have their sockets in the premaxillary bone ; they are generally chisel-shaped, and with their opponents of the lower jaw act like scissors. The canine tooth is the first tooth behind the premaxillo-maxillary suture, provided it be not far behind it; it is almost always the first of the premaxillary series, speaking accurately, which is elongated and sharply pointed. The premolar teeth are those in the maxillary bone which are preceded by milk teeth. The molar teeth are those, behind the premolars, which are not preceded by temporary teeth.