1. premaxillae.12. squamosal.
2. maxillae.13. supratemporal.
3. nasal.14. quadratojugal.
4. prefrontal[74].15. sclerotic ring.
5. frontal.16. postorbital.
6. postfrontal[74].17. jugal.
7. anterior nares.18. lachrymal.
8. orbit.19. dentary.
9. supratemporal fossa.20. articular.
10. interparietal foramen.21. angular.
11. parietal.

The ribs are long, and the anterior ones have capitula and tubercula. There is no sternum, but the ventral body wall is strengthened by a complex system of abdominal splint ribs.

The pectoral girdle is strongly developed, the scapulae are narrow, the coracoids broad, and meet ventrally without overlapping. There are probably no precoracoids, but clavicles and a T-shaped interclavicle are well developed.

The limbs are very short, and completely modified into swimming paddles. The humerus and femur are both short, while the radius and ulna, tibia and fibula are generally still further reduced to the form of short polygonal bones.

The digits are formed of longitudinal series of very numerous small bones. The number of digits is five, but there sometimes appear to be more owing to the bifurcation of certain of them, or to the addition of marginal bones, either to the radial or ulnar side of the limb. The humerus has no foramen, and both humerus and femur are unique in that they are distally terminated by concave surfaces instead of by convex condyles. The pelvic limb is much smaller than the pectoral. The pelvis has no bony connection with the vertebral column, and all the component bones are small and rod-like.

The Ichthyosauria are confined to beds of Secondary age and by far the best known genus is Ichthyosaurus.

Order 5. Rhynchocephalia.

This order includes the living Sphenodon (Hatteria) and various extinct forms. The general shape of these animals is lizard-like and the tail is long.

The vertebrae are amphicoelous or sometimes nearly flat, and the notochord sometimes persists to some extent. Proterosaurus differs from the other members of the order in having opisthocoelous cervical vertebrae.

The sacrum is composed of two vertebrae. Ossified inter centra (interdorsalia) generally occur in the cervical and caudal regions, and sometimes throughout the whole vertebral column. In the skull the quadrate is immovably fixed and united to the pterygoid. The palate is well ossified, while the premaxillae which are often beak-like are never ankylosed together. The jaws may be toothless or may be provided with teeth which are usually acrodont (see p. 199). The palatines frequently bear teeth, and in Proterosaurus teeth occur also on the pterygoids and vomers. The rami of the mandible are united by ligament at the symphysis except in the Rhynchosauridae, in which the union is bony. Superior and inferior temporal arcades occur.