In this group the incisors are small, subequal and numerous, not less than 4/3. The canines are larger than the incisors, and the molars have sharp cusps. The members of this group are all more or less carnivorous or insectivorous. The group includes the families Didelphyidae, Dasyuridae, Peramelidae, and Notoryctidae[117].
2. Diprotodontia.
In this group the incisors do not exceed 3/3, and are usually 3/1, occasionally 1/1. The first upper and lower incisors are large and cutting. The lower canines are always small or absent, and so in most cases are the upper canines. The molars have bluntly tuberculated, or transversely ridged crowns. The group includes the families Phascolomyidae, Phalangeridae, Macropodidae, and Epanorthidae.
Subclass III. Monodelphia or Eutheria.
This great group includes all the Mammalia except the orders Monotremata and Marsupialia. Coming to their general characteristics—as in the Didelphia the odontoid process and cervical ribs early become fused with the centra which bear them, while the coracoid is reduced so as to form a mere process on the scapula, and there is no precoracoid (epicoracoid), such as is found in Ornithodelphia. Clavicles may be present or absent; when fully developed they articulate with the sternum, usually directly, but occasionally, as in some Rodents and Insectivores, through the remains of the sternal end of the precoracoid. There is never any interclavicle in the adult, though sometimes traces of it occur during development. In the pelvis the acetabula are imperforate; and well-developed epipubic bones are never found in the adult, though traces of them occur in some Carnivores and foetal Ungulates.
Order 1. Edentata[118].
Teeth are not, as the name of the order seems to imply, always wanting; and sometimes they are very numerous. They are, however, always imperfect, and, with very few exceptions, are homodont and monophyodont. They have persistent pulps, and so grow indefinitely and are never rooted. In all living forms they are without enamel, consisting merely of dentine and cement, and are never found in the front part of the mouth in the situation occupied by the incisors of other mammals. These characters derived from the teeth are the only ones common to the various members of the order, which includes the living sloths, ant-eaters, armadillos, pangolins and aard varks, together with various extinct forms, chiefly found in beds of late tertiary age in both North and South America, the best known being the Megatheridae and Glyptodonts.
Order 2. Sirenia[119].
The skeleton of these animals has a general fish-like form, in correlation with their purely aquatic habits. The fore limbs have the form of paddles, but the number of phalanges is not increased beyond the normal. There are no external traces of hind limbs.
The whole skeleton and especially the skull and ribs is remarkably massive and heavy. The dentition varies; in the two living genera Manatus and Halicore, incisor and molar teeth are present, in one extinct genus, Rhytina, teeth are entirely absent, while in another, Halitherium, the dentition is more decidedly heterodont than in living forms. In the two living genera the dentition is monophyodont, but in Halitherium the anterior grinding teeth are preceded by milk teeth. The tongue and anterior part of the palate and lower jaw are covered with roughened horny plates. The skull is noticeable for the size and backward position of the anterior nares, also for the absence or small size of the nasal bones. There is no union of certain of the vertebrae to form a sacrum, and in living forms the centra are not terminated by well-formed epiphyses[120].