The order Monotremata includes only two living families, the Echidnidae and Ornithorhynchidae.
Mesozoic Mammalia[113].
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].
The odontoid process at an early stage becomes fused with the centrum of the second cervical vertebra, and the number of thoraco-lumbar vertebrae is always nineteen. The skull has several characteristic features. The tympanic bone remains permanently distinct, and the anterior boundary of the tympanic cavity is formed by the alisphenoid. The carotid canal perforates the basisphenoid, and the lachrymal canal opens either outside the orbit or at its margin. There are generally large vacuities in the palate. The angle of the mandible is (except in Tarsipes) more or less inflected; and as a rule the jugal furnishes part of the articular surface for the mandible. There is no precoracoid (epicoracoid) or interclavicle, and the coracoid is reduced to form a mere process of the scapula, not coming near the sternum.
Epipubic, or so-called marsupial bones[116], nearly always occur, and a fourth pelvic element, the acetabular bone, is frequently developed. The fibula is always complete at its distal end, sometimes it is fused with the tibia, but often it is not only free but is capable of a rotatory movement on the tibia. This is the case in the families Phascolomyidae, Didelphyidae, and Phalangeridae.
The Marsupialia can be subdivided into two main groups, according to the character of the teeth:—
1. Polyprotodontia.