The simplest dentition is that of many species of dolphin (fig. 1), in which the crowns are single-pointed, slightly curved cones, and the roots also single and tapering; so that all the teeth are alike in form from the anterior to the posterior end of the series, though it may be with some slight difference in size, those at the two extremities being rather smaller than the others. Such a dentition is called "homoeodont" (Gr. [Greek: homoios], like, [Greek: odous], tooth), and in the case cited, as the teeth are never changed, it is also monophyodont (Gr. [Greek: monos], alone, single, [Greek: phyein], to generate, [Greek: odous], tooth). Such teeth are adapted only for catching slippery living prey, like fish.
In a very large number of mammals the teeth of different parts of the series are more or less differentiated in character; and, accordingly, have different functions to perform. The front teeth are simple and one-rooted, and are adapted for cutting and seizing. They are called "incisors." The back, lateral or cheek teeth, on the other hand, have broader and more complex crowns, tuberculated or ridged, and supported on two or more roots. They crush or grind the food, and are hence called "molars." Many mammals have, between these two sets, a tooth at each corner of the mouth, longer and more pointed than the others, adapted for tearing or stabbing, or for fixing struggling prey. From the conspicuous development of such teeth in the Carnivora, especially the dogs, they have received the name of "canines." A dentition with its component parts so differently formed that these distinctive terms are applicable to them is called heterodont (Gr. [Greek: heteros], different). In most cases, though by no means invariably, mammals with a heterodont dentition are also diphyodont (Gr. [Greek: diphyês], of double form).
[Illustration: FIG. 2.--Milk and Permanent Dentitions of Upper (I.) and Lower (II.) Jaws of the Dog (_Canis_), with the symbols by which the different teeth are designated. The third upper molar (_m_ 3) is the only tooth wanting to complete the typical heterodont mammalian dentition.]
This general arrangement is obvious in a considerable number of mammals; and examination shows that, under great modifications in detail, there is a remarkable uniformity of essential characters in the dentition of a large number of members of the class belonging to different orders and not otherwise closely allied, so much that it is possible to formulate a common plan of dentition from which the others have been derived by the alteration of some and the suppression of other members of the series, and occasionally, but very rarely, by addition. In this generalized form of mammalian dentition the total number of teeth present is 44, or 11 above and 11 below on each side. Those of each jaw are placed in continuous series without intervals between them; and, although the anterior teeth are simple and single-rooted, and the posterior teeth complex and with several roots, the transition between the two kinds is gradual.