The foramen magnum forms a large round hole leading into the cranial cavity, and is bounded laterally by the two exoccipitals and below by them, and to a very slight extent by the basi-occipital, the three bones together forming a concave occipital condyle by which the vertebral column articulates with the skull.

The exoccipitals are connected laterally with a fourth pair of auditory bones, the opisthotics, and just meet the epi-otics dorsolaterally, while dorsally they are separated by a wide tract of unossified cartilage from the supra-occipital.

The opisthotics are connected laterally with the pterotics.

Side of the Cranium.

At the posterior end is seen the basi-occipital in contact above with the exoccipital, which is pierced by a prominent foramen for the exit of the tenth nerve. In front of this lies a small foramen, sometimes double, for the ninth nerve.

[a]Fig. 10. Lateral view of the chondrocranium of a Salmon]
(Salmo salar) (after [a]Parker]). A few membrane bones are also shown.
Cartilage is dotted.

1. supra-occipital.12. olfactory pit; the vomerine
2. epi-otic.teeth are seen just below.
3. pterotic.14. pro-otic.
4. opisthotic.15. basisphenoid.
5. exoccipital.16. foramen for the passage of
6. basi-occipital.an artery.
7. parasphenoid.17. anterior fontanelle.
8. sphenotic.18. posterior fontanelle.
9. alisphenoid.I. II. V. VII. IX. X. foramina
10. orbitosphenoid.for the passage of cranial nerves.
11. lateral- or ectethmoid.

In front of the exoccipital is the large pro-otic pierced by two prominent foramina. Through the more dorsal of these (fig. 10, VII.) the facial nerve passes out, while the more ventral (fig. 10, 16) is for the passage of an artery. Dorsal to the exoccipital are the opisthotic and pterotic, and dorsal to the pro-otic is the sphenotic. The pterotic is marked by a prominent groove often lined by cartilage, which is continued forwards along a tract of cartilage between the pro-otic and sphenotic. With this groove the hyomandibular articulates.

There are considerable ossifications in the sphenoidal region of the side of the cranium. The anterior boundary of the posterior fontanelle is formed by the large alisphenoid, which is continuous behind with the pro-otic and sphenotic, and below with a slender basisphenoid. Both in front of and behind the basisphenoid there are considerable vacuities in the walls of the cranium; through the posterior of these openings (fig. 10, V.) the main part of the trigeminal nerve passes out, and through the anterior one, the optic (fig. 10, II.). The alisphenoid is continuous in front with the orbitosphenoid (fig. 10, 10), which is pierced by the foramen for the exit of the first nerve (fig. 10, I.), and in front of the orbitosphenoid there is a large vacuity. The lateral ethmoid is seen in the side view as well as in the dorsal view. Further forwards are seen the olfactory pits, and the long cartilaginous snout.