The conceptual aspects of the significance and possible mechanisms of production of variations in size, number and shape of teeth have been elaborated in the General introduction. In general, smallness of teeth, reduction in their complexity and total absence are different degrees of the same thing, explicable in terms of the morphogenetic field concept.

In this subdivision of the book, composed of six chapters, each of which is devoted to a different mammalian order or group of orders, examples will be systematically described. Much of the account has to be anecdotal, that is it consists of a catalogue of examples, but wherever possible the information is collated in order to make generalizations.

For some supernumerary teeth, it is not possible to record more than that they were present and where they were situated. Their shape, whether they were simple, conical or tuberculate in their morphology, or whether they resembled teeth of the normal series (supplemental teeth, Chapter 1, p. 6) is often not recorded. Where, for example, the primary account mentions that an extra premolar was present, it is not always clear whether it was so designated because it was in the premolar region of the arch, or whether it had the morphology of a premolar, in which case, of course, there would be difficulty in deciding which of the premolars was supernumerary. It is unusual for such teeth to resemble those of the normal series exactly, so they are often recognizable by some degree of abnormal morphology.