The cartilage bones of the cranium proper are the two exoccipitals and the sphenethmoid.

The exoccipitals (figs. 24, 25, and 26, 6) are a pair of irregular bones bounding the foramen magnum at the posterior end of the skull. They almost completely surround the foramen magnum, and bear a pair of oval convex surfaces, the occipital condyles, with which the first vertebra articulates. The bones generally called the exoccipitals of the frog include the epi-otic and opisthotic elements of many skulls, in addition to the exoccipitals.

Fig. 24. A dorsal, and B ventral views of the cranium of a Common Frog (Rana temporaria) × 2 (after Parker). In this and the next two figs. cartilage is dotted, cartilage bones are marked with dots and dashes, membrane bones are left white.

1. sphenethmoid.14. vomer.
2. fronto-parietal.15. posterior nares.
3. pterygoid.16. palatine.
4. squamosal.18. columella.
6. exoccipital.19. quadrate.
7. parasphenoid.20. occipital condyle.
8. pro-otic.II. optic foramen.
9. quadratojugal.V. VII. foramen for exit of
10. maxillae.trigeminal and facial nerves.
11. nasal.IX. X. foramina for exit of
12. premaxillae.glossopharyngeal and
13. anterior nares.pneumogastric nerves.

The patch of unossified cartilage immediately external to the occipital condyle is pierced by two small foramina, through which the ninth and tenth nerves leave the cranial cavity. The ninth nerve passes through the more external of these foramina, the tenth through the one nearer the condyle. The foramina lie however very close together and are sometimes confluent. The cranial walls for a considerable distance in front of the occipitals are unossified, but the anterior end of the cranial cavity is encircled by another cartilage bone, the sphenethmoid (figs. 24 and 25, 1) or girdle bone. This partly corresponds to the orbitosphenoids of the Newt's skull. Anteriorly it is pierced by a pair of small foramina through which the ophthalmic branches of the trigeminal nerve pass out.

The anterior part of the cranial cavity is divided into two halves by a vertical plate, the mesethmoid. Some little distance behind the sphenethmoid the ventro-lateral walls of the cartilaginous cranium are pierced by a pair of rather prominent holes, the optic foramina (figs. 24 and 25, B, II), and at a similar distance further back, occupying a kind of notch in the pro-otic are the large trigeminal foramina, through which the fifth and seventh nerves leave the cranium. Between the trigeminal and optic foramina are the very small foramina for the sixth nerves (fig. 25, B, VI).

The membrane bones of the cranium proper include the fronto-parietals and the parasphenoid.

The fronto-parietals (figs. 24 and 26, A, 2) form a pair of long flat bones closely applied to one another in the middle line, the line of junction being the sagittal suture. They cover over the fontanelles and overlap the sphenethmoid in front.

The parasphenoid (figs. 24 and 26, B, 7) is a bone shaped like a dagger with a very short handle. It lies on the ventral surface of the cranium, the blade being directed forwards and underlying the sphenethmoid; its lateral processes underlie the auditory capsules.