SYSTEM.] MAMMALIA 353 three anterior teeth from the others. These three teeth then, which are implanted by their roots in the pre- rnaxilla, form a distinct group, to which the name of " incisor " is applied. This distinction is, however, not so important as it appears at first sight, for, as mentioned when speaking of the development of the teeth, their connexion with the bone is only of a secondary nature, and, although it happens conveniently for our purpose that in the great majority of cases the segmentation of the bone coincides with the interspace between the third and fourth tooth of the series, still, when it does not happen to do so, as in the case of the Mole, we must not give too much weight to this fact, if it contravenes other reasons for determining the homologies of the teeth. The eight remaining teeth of the upper jaw offer a natural division, inasmuch as the posterior three never have milk predecessors, and, although some of the anterior teeth may be in the same case, the particular one pre ceding these three always has such a predecessor. These three then are grouped apart as the "molars," or "true molars," as some of the teeth in front of them often have a molariform character. Of the five teeth between the incisors and molars the most anterior, or that which is usually situated close behind the premaxillary suture, almost always, as soon as any departure takes place from the simplest and most homogeneous type, assumes a lengthened and pointed form, and is the tooth so developed as to constitute the " canine " or "laniary" tooth of the Car- nivora, the tusk of the Boar, &c. It is customary therefore to call this tooth, whatever its size or form, the " canine." The remaining four are the " premolars " or " false molars." This system of nomenclature has been objected to as being artificial, and in many cases not descriptive, the distinction between premolars and canine especially being sometimes not obvious, but the terms are now in such general use, and are so practically convenient especially if, as it is best to do in all such cases, we forget their original signification, and treat them as arbitrary signs that it is not likely they will be superseded by any that have been proposed as substitutes for them. With regard to the lower teeth the difficulties are greater, owing to the absence of any suture coi-responding to that which defines the incisors above ; but, as the number of the teeth is the same, as the corresponding teeth are preceded by milk teeth, and as in the large majority of cases it is the fourth tooth of the series which is modified in the same way as the canine (or fourth tooth) of the upper jaw, it is quite reasonable to adopt the same divisions as with the upper series, and to call the first three, which are implanted in the part of the mandible opposite to the pre- maxilla, the incisors, the next the canine, the next four the premolars, and the last three the molars, It may be observed that when the mouth is closed, especially when the opposed surfaces of the teeth present an irregular out line, the corresponding upper and lower teeth are not exactly opposite, otherwise the two series could not fit into one another, but as a rule the points of the lower teeth shut into the interspaces in front of the corresponding teeth of the upper jaw. This is seen very distinctly in the canine teeth of the Carnivora, and is a useful guide in determining the homologies of the teeth of the two jaws. Objections have certainly been made to this view, because, in certain rare cases, the tooth which, according to it, would be called the lower canine has the form and function of an incisor (as in Ruminants and Lemurs), and on the other hand (as in Oreodon, an extinct Ungulate from North America) the tooth that would thus be determined as the first premolar has the form of a canine ; but it should not be forgotten that, as in all such cases, definitions derived from form and function alone are quite as open to objection as those derived from position and relation to sur rounding parts, or still more so. TO.1 nv g m.3 FIG. 3. Milk and Permanent Dentition of Upper (I.) and Lower (II.) Jaw of the Dog (Cam s familiaris), with the symbols by which the different teeth are commonly designated. The third upper molar (m 3) is the only tooth wanting in this animal to complete the typical heterodont mammalian dentition. For the sake of brevity the complete dentition, arranged Dental according to these principles, is often described by the fol- formulae- lowing formula, the numbers above the line representing the teeth of the upper, those below the line those of the lower jaw: incisors ~ y canines ^~ v premolars j^j, molars ij ; total 44. As, however, initial letters may be substituted for the names of each group, and it is quite unnecessary to give more than the numbers of the teeth on one side of the mouth, the formula may be conveniently abbreviated into
- i c T P t m I = il ; total 44 -
The individual teeth of each group are always enumerated from before backwards, and by such a formula as the following i 1, i 2, i 3, c, p 1, p 2, p 3, p 4, m 1, m 2, m 3 i 1, i 2, i 3, c, p 1, p 2, p 3, p 4, m 1, m 2, m 3 ~ a special numerical designation is given by which each one can be indicated. In mentioning any single tooth, such a sign as will mean the first upper molar, ml the first lower molar, and so on. The use of such signs saves much time and space in description. It was part of the view of the founder of this system of dental notation that, at least throughout the group of mammals whose dentition is derived from this general type, each tooth has its strict homologue in all species, and that in those cases in which fewer than the typical number are present (as in all existing mammals except the genera Sus, Gymnura, Talpa, and Myogale) the teeth that are missing can be accurately defined. According to this view, when the number of incisors falls short of three it is assumed that the absent ones are missing from the outer and posterior end of the series. Thus when there is but one incisor present, it &i; when two, they are i 1 and i 2. Furthermore, when the premolars and the molars are below their typical number, the absent teeth are missing from the fore part of the premolar series, and from the back part of the molar series. If this were invariably so, the labours of those who describe teeth would be greatly simplified; but there are unfortun ately so many exceptions that a close scrutiny into tha
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