The Chondrocranium.
This is an elongated structure, wide behind owing to the fusion of the large auditory capsules with the cranium, and elongated and tapering considerably in front; in the middle it is much contracted by the large orbital cavities.
Dorsal surface of the Cranium.
In the centre of the posterior end of the dorsal surface is the supra-occipital (fig. 9, A, 1) with a prominent posterior ridge. It is separated by two tracts of unossified cartilage from the large series of bones connected with the auditory organ. The first of these is the epi-otic (fig. 9, 2), which is separated by only a narrow tract of cartilage from the supra-occipital, and is continuous laterally with the large pterotic (fig. 9, A, 3) which overlaps in front a smaller bone, the sphenotic (fig. 9, 4). Both epi-otic and pterotic are drawn out into rather prominent backwardly-projecting processes.
[a]Fig. 9. A. dorsal and B. ventral view of the cranium of a Salmon]
(Salmo salar) from which most of the membrane bones have been
removed (after [a]Parker]). Cartilage is dotted.
| 1. supra-occipital. | 12. opisthotic. |
| 2. epi-otic. | 13. alisphenoid. |
| 3. pterotic. | 14. orbitosphenoid. |
| 4. sphenotic. | 16. foramen for passage of an |
| 5. frontal. | artery. |
| 6. median ethmoid. | 17. pro-otic. |
| 7. parietal. | 18. articular surface for |
| 8. lateral ethmoid. | hyomandibular. |
| 9. parasphenoid. | II. VII. IX. X. foramina for the |
| 10. vomer. | passage of cranial nerves. |
| 11. exoccipital. |
The greater part of the remainder of the dorsal surface is formed of unossified cartilage which is pierced by three large vacuities or fontanelles. The anterior fontanelle is unpaired, and lies far forward near the anterior end of the long cartilaginous snout, the two larger posterior ones lie just in front of the supra-occipital and lead into the cranial cavity. In front of the orbit the skull widens again, and is marked by two considerable lateral ethmoid (fig. 9, 8) ossifications. In front of these are a pair of deep pits, the nasal fossae, at the base of which are a pair of foramina through which the olfactory nerves pass out; they communicate with a space, the middle narial cavity, seen in a longitudinal section of the skull.
The long cartilaginous snout is more or less bifid in front, especially in the male (fig. 9).
Posterior end of the Cranium.