A special numerical designation is thus given by which each one can be indicated. In mentioning any single tooth, such a sign as _m¹_⁄ will mean the first upper molar, ⁄_m₁_ the first lower molar, and so on. The use of such signs saves much time and space in description.[6] 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 is _i_ 1; 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 so many exceptions that a close scrutiny into the situation, relations, and development of a tooth is required before its nature can be determined, and in some cases the evidence at our disposal is scarcely sufficient for the purpose. In other instances, however, as among the Polyprotodont Marsupials, we have decisive evidence to show that the missing premolar teeth are not those at the extremity of the series. [Illustration: FIG. 3.—Milk and Permanent Dentition of Upper (I.) and Lower (II.) Jaw of the Dog (_Canis 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.]