The most salient "mammalian" feature of the Theriodonts is the heterodonty of the teeth, the pattern of the "molars," and the limited number which constitute the series. The fact, too, that they are limited to the dentary bones below and to the {92} maxillae and the premaxillae above, is a _sine qua non_ for mammalian comparison. In the more basal Theromorpha the teeth are not so limited in position. Finally, to complete the remarkable mammalian resemblance of the teeth of these reptiles, it must be mentioned that in _Tritylodon_ and _Diademodon_ the roots of the molars, as we may fairly term them, though not actually divided after the mammalian fashion, were deeply marked by a groove, which suggests an incipient division or a fusion of two distinct roots. Some of these facts of structure may now be considered in further detail. As to the incisors and canines, it is sufficient to say that the numbers of the former, and the shape of the latter, are in perfect consonance with a derivation of the Mammalia from this group. The molar series can be divided into premolars and molars, at least in so far as regards their shape; for the anterior teeth are often smaller and less complicated than those which follow, as is often the case with the two series in mammals. The molar series also consist of teeth in close apposition to each other and separated from the canines by a diastema, which is a character of mammalian teeth. The fact that in the reptile _Cynognathus_ and the mammal _Myrmecobius_ there are nine of these molar teeth in each half of each jaw is perhaps not a point upon which it is desirable to dwell with too much weight; but the general fact that the molars are further reduced in some genera of Theriodontia than in that which has been mentioned, is clearly a matter of significance when the ancestry of the mammals is under consideration.