This subclass contains only a single order, the Monotremata, and the following characteristics are equally applicable to the subclass and to the order. The vertebral centra have no epiphyses, and the odontoid process remains for a long time free from the centrum of the second vertebra. With the exception of the atlas of _Echidna_ the cervical vertebrae are without zygapophyses. The cranial walls are smooth and rounded, and the sutures between the several bones early become completely obliterated as in birds. The mandible is a very slight structure, with no ascending ramus, and with the coronoid process (see p. 398) and angle rudimentary. The auditory ossicles show a low state of development. The tubercula of the ribs articulate with the sides of the centra of the thoracic vertebrae, not with the transverse processes. Some of the cervical ribs remain for a long time separate from the vertebrae. Well ossified sternal ribs occur. No true teeth are present in the adult. The young _Ornithorhynchus_ has functional molar teeth, but in the adult their place is taken by horny plates. In the Echidnidae neither teeth nor horny plates occur. The coracoid (fig. 66, 3) is complete and well developed, and articulates with the sternum. A precoracoid (epicoracoid) occurs in front of the coracoid, and there is a large interclavicle (fig. 66, 6). The ridge on the scapula, corresponding to the spine of other mammals, is situated on the anterior border instead of in the middle of the outer surface. Epipubic bones are present. In the Echidnidae, but not in _Ornithorhynchus_[112], the central portion of the acetabulum is unossified as in birds. The humerus has a prominent deltoid crest; its ends are much expanded, and the distal end is pierced by an ent-epicondylar foramen. The fibula has a broad proximal process resembling an olecranon. The limbs and their girdles bear a striking resemblance to those of some Theromorphous reptiles. [Illustration FIG. 66. VENTRAL VIEW OF THE SHOULDER-GIRDLE AND STERNUM OF A DUCKBILL (_Ornithorhynchus paradoxus_) × 3/4 (after PARKER). 1 and 2. scapula. 3. coracoid. 4. precoracoid (epicoracoid). 5. glenoid cavity. 6. interclavicle. 7. clavicle. 8. presternum. 9. third segment of mesosternum. 10. sternal rib. 11. intermediate rib. 12. vertebral rib.] The order Monotremata includes only two living families, the Echidnidae and Ornithorhynchidae. MESOZOIC MAMMALIA[113].