A common form in the Purbeck of Dorsetshire is _Triconodon_ (_Triacanthodon_), in which the formula of the lower teeth is _i_ 3, _c_ 1, _p_ 4, _m_ 3-4. A lower jaw is shown in Fig. 28, and an enlarged view of a molar tooth in Fig. 4 (5). The molar teeth consist of three flattened cones placed in the same antero-posterior line, those of the upper and lower jaw being alike. _Priacodon_, of the Jurassic of the United States, is probably inseparable from _Triconodon_. In the genus _Phascolotherium_ (Fig. 29) of the Lower Jurassic Stonesfield Slate, the lower teeth may be classified as _i_ 4, _c_ 1, _p_ 3, _m_ 4, the premolars and molars being much alike. The molars approximate to the type of those of _Triconodon_, but the anterior and posterior cones are relatively smaller. Like that of the last-named genus, the mandible of _Phascolotherium_ is remarkable for the extremely low position of its articular condyle. In _Amphilestes_ (Fig. 30) of the Stonesfield Slate the molars appear to be of the same general type as those of _Phascolotherium_, but are more numerous, although their exact number cannot be determined. A somewhat different type of lower molar is displayed by the genus _Amblotherium_, of the Dorsetshire Purbeck, to which _Amphitherium_ of the Stonesfield Slate was probably allied. This type of tooth is shown in Fig. 4 (8, 9, 12) p. 31, and, as there stated, represents that modification of the tritubercular type known as the tubercular sectorial. The three primitive tritubercular cusps form what is known as the blade of the tooth, behind which there is the talon or hypocone. A similar form of molar occurs in the existing Opossums and Bandicoots. The number of lower teeth in _Amblotherium_ is _i_ 4, _c_ 1, _p_ 4, _m_ 7-8. Numerous allied types, such as _Achyrodon_ and _Dryolestes_ occur in the Upper Jurassic of Europe or the United States, while from only one side of the jaw being exposed in each case so-called genera like _Stylodon_ and _Stylacodon_ have been formed upon specimens showing the opposite side to that which is exposed in the types of _Amblotherium_ and _Amphitherium_. The only parallel among existing forms to the excessive number of molar teeth found in these Mesozoic genera occurs in the Marsupial genus _Myrmecobius_, of which a description is given in a succeeding chapter. Jaws more or less closely resembling those described under the names mentioned above are also found in the uppermost Cretaceous of the United States. A feature common to these Mesozoic mammals and _Myrmecobius_ and some other existing forms is the presence of a narrow channel on the inner side of the mandibular ramus known as the mylohyoid groove (Fig. 29).