In front of the premolar teeth, and between them and the canine, if it be present, or the incisors, if it be absent, there is often a space called the "diastema." It is best marked in the Rodentia and Ungulata, and in the horse is familiar as the place where the bit lies. The elephant's tusks are modified incisors, those of the walrus and wild boar are modified canines.
Dental Formulae.—In recording the teeth of any particular mammal it saves time and space if a dental formula be used. This simply means setting down the number of each kind of tooth in'one side of the upper and lower jaw in their order from before backward. Thus man's formula would be, incisors canines premolars molars 3 • This is condensed into 2 *I .2 I 2 3 2.1.2.3 Some other types of dental formulae are 14. Pleurodont, a tooth anchylosed to the inner side of a para pet on the jaw.
16. Polyphyodont, having an endless succession of teeth, as in most vertebrates below the mammals.
17. Polyprotodont, a marsupial having an incisor formula of more than -I.
18. Protodont, a stage met with in fossil mammals which is an advance on the haplodont tooth in that two small cusps are added to the main cone.
19. Secodont, a back tooth adapted to cutting, as in many of the Carnivora.
20. Selenodont, a molar tooth with crescentic ridges on its grinding surface, as in most ruminants.
21. Thecodont, a tooth embedded in a socket or alveolus, as in mammals.
2 2. Triconodont, a fossil stage in advance of the protodont. There are three well-marked cones in an antero-posterior line.