(3) The jaws.

The upper jaw consists of a rod of cartilage connected with the cranium near its two ends, but widely separated from it in the middle. It is almost completely overlain by membrane bone. With its posterior end the lower jaw articulates.

The membrane bones of the upper jaw include first the premaxillae, a small bone meeting its fellow in the middle line, and forming the extreme anterior end of the upper jaw. It gives off on its dorsal surface a backwardly-projecting process. It is connected behind with the maxillae (figs. 24 and 26, 10), a long flattened bone which forms the greater part of the margin of the upper jaw, and gives off near its anterior end a short process which projects upwards and meets the nasal.

Fig. 26. A, Lateral view of the skull, B, Posterior view of the cranium of a Common Frog (Rana temporaria) × 2 (after Parker).

1. sphenethmoid.14. mento-meckelian.
2. fronto-parietal.15. dentary.
3. pterygoid.16. angulo-splenial.
4. squamosal.17. basilingual plate.
5. tympanic membrane.19. quadrate.
6. exoccipital.20. columella.
7. parasphenoid.21. occipital condyle.
8. pro-otic.22. anterior cornu of the hyoid
9. quadratojugal.(cerato-hyal).
10. maxillae.23. foramen magnum.
11. nasal.II. IX. X. foramina for the exit
12. premaxillae.of cranial nerves.
13. anterior nares.

Both maxillae and premaxillae are grooved ventrally, and bear, attached to the outer more prominent margin of the groove, a row of minute conical teeth. These teeth are pleurodont, that is, are ankylosed by their bases and outer sides to the margin of the jaw. Each tooth is a hollow cone, the basal part of which is formed of bone, the apical part of dentine, capped by a very weak development of enamel.

The posterior end of the maxillae is overlapped by a small bone, the quadratojugal (figs. 24 and 26, 9), whose posterior end forms part of the articular surface for the lower jaw. Just behind the quadratojugal there is a small unossified area which lies at the angle of the mouth, and is connected by a narrow bar of cartilage with the cranium; this forms the quadrate (figs. 24 and 26, 19). A backwardly-directed outgrowth from the cartilaginous bar more or less completely surrounds the tympanic membrane, forming the tympanic ring. When followed back the maxillae and quadratojugal diverge further and further from the cranium, till the angle of the mouth comes to be separated from the foramen magnum by a space nearly double the width of the cranium. This space is bridged over to a considerable extent by two triradiate bones, the pterygoid and squamosal.

The pterygoid (figs. 24 and 26, 3) is a large bone, whose anterior limb runs forwards meeting the maxillae and palatine; while its inner limb meets the auditory capsule and parasphenoid, and its outer limb runs backwards and outwards to the angle of the mouth. The palatine is a small transversely-placed bone, which connects the pterygoid with the anterior part of the sphenethmoid. The squamosal (figs. 24 and 26, 4) is a T-shaped bone whose anterior arm is pointed and passes forwards to meet the pterygoid. The posterior upper arm is closely applied to the pro-otic, while the posterior lower arm meets the pterygoid and quadratojugal at the angle of the jaw, and surrounds the narrow cartilaginous bar of the quadrate which goes to join the cranium. The squamosal is probably homologous with the squamosal together with the pre-opercular of Bony Ganoids.

The quadrate and squamosal form the suspensorium by which the lower jaw is connected with the cranium.