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. 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.