Bilophodont to teeth marked by a simple pair of transverse ridges, with or without a third ridge running along the outer border of the tooth at right angles to the other two, e.g. the grinding teeth of Lophiodon, Kangaroo, Manatee, Tapir, Dinotherium;

Selenodont to teeth marked by crescentic ridges running from the anterior towards the posterior end of the tooth, e.g. the grinding teeth of the Ox and Sheep.

Teeth whose crowns are low so that their whole structure is visible from the grinding surface are called brachydont, while those with higher crowns, in which the bases of the infoldings of enamel are invisible from the grinding surface are said to be hypsodont. Bunodont teeth are brachydont, the teeth of the Horse and Ox are hypsodont.

Passing now to the appendicular skeleton—the shoulder girdle differs markedly from that of Sauropsids in the fact that the coracoid, except in the Ornithodelphia, is greatly reduced, generally forming only a small process on the scapula. In the pelvis the pubes meet in a ventral symphysis, except in some Insectivora and Chiroptera. In many mammals a fourth pelvic element, the acetabular bone, is distinguishable. The ankle joint is cruro-tarsal, or situated between the proximal tarsal bones and the tibia and fibula. Carpalia 4 and 5 are united forming the unciform; and the ulnar sesamoid bone or pisiform is generally well developed. In the proximal row of tarsal elements there are only two bones, the calcaneum and astragalus. Of these the calcaneum is the fibulare, and the astragalus is generally regarded as the tibiale and intermedium fused[111].

Subclass I. Ornithodelphia or Prototheria.

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.

Fig. 66. Ventral view of the shoulder-girdle and sternum of a Duckbill (Ornithorhynchus paradoxus) × ¾ (after Parker).

1 and 2. scapula.8. presternum.
3. coracoid.9. third segment of
4. precoracoid (epicoracoid).mesosternum.
5. glenoid cavity.10. sternal rib.
6. interclavicle.11. intermediate rib.
7. clavicle.12. vertebral rib.