It will be well here to briefly refer to certain mammals of small size, the remains of which have been found in deposits of Mesozoic age. In the great majority of cases they are known only by the lower jaw, or sometimes only by isolated teeth. A large number of them are commonly grouped together as the Multituberculata, and are sometimes, partly owing to the resemblance of their teeth to those of _Ornithorhynchus_, placed with the Prototheria, sometimes between the Prototheria and the Metatheria. They are characterised by having a single pair of large incisors in the lower jaw, and one large with one or two smaller incisors in each premaxillae. The lower canines are very small or altogether wanting. The incisors are separated by a diastema from the grinding teeth, which are sometimes (_Tritylodon_) characterised by the possession of longitudinal rows of little tubercles separated by grooves, sometimes by having the premolars provided with high cutting edges, whose surfaces are obliquely grooved. Some of the Mesozoic mammals found associated with the Multituberculata, have however a dentition of an altogether different type, with at least three lower incisors, well developed canines and premolars, and numerous molars with peculiar three-cusped or tritubercular grinding surfaces. These mammals, one of the best known of which is _Phascolotherium_, are commonly separated from the Multituberculata, and are divided by Osborn into two groups, one allied to the Marsupials, and one to the Insectivores. The group showing Marsupial affinities is further subdivided into carnivorous, omnivorous, and herbivorous subgroups. The members of both groups commonly have four premolars, and six to eight molars in each mandibular ramus. _Subclass II._ DIDELPHIA OR METATHERIA. This subclass, like the previous one, contains only a single order, viz. the Marsupialia[114]; but the forms referable to it are far more numerous than in the case of the Monotremata. The integument is always furry, and the teeth are always differentiated into incisors, canines, premolars and molars. Except in _Phascolomys_, the number of incisors in the upper and lower jaws is never equal, and the number in the upper usually exceeds that in the lower jaw. There is no such regular succession and displacement of teeth as in most mammals. Sometimes the anterior teeth are diphyodont, and as a general rule the tooth commonly regarded as the last premolar is preceded by a milk tooth. The majority of the permanent teeth of most Marsupials are regarded as belonging to the milk series for two reasons, (1) they are developed from the more superficial tissues of the jaws, (2) a second set, the permanent teeth, begin to develop as outgrowths from them, but afterwards become aborted[115].