The Vertebrate Skeleton

by Sidney H. Reynolds

At the posterior end of the dorsal surface is the large supra-occipital, which is drawn out behind into the large blade-like occipital spine. On each side of the supra-occipital are the small irregular parietals, while in front of it the roof of the skull is mainly formed by the very large unpaired frontal.

A complicated series of bones are developed in connection with the auditory capsule, which forms a large projecting mass united with the side of the cranium and drawn out behind into a pair of strong processes, the epi-otic and parotic processes. Both these processes are connected behind with a large V-shaped bone, the post-temporal (fig. 13, 1), which will be described when dealing with the pectoral girdle. The epi-otic process is formed by the epi-otic, which is continuous in front with the parietal. The parotic process is formed by two larger bones, a more dorsal one, the pterotic, and a more ventral and internal one, the opisthotic, which is continuous in front with the large pro-otic. Intervening between the pterotic and frontal is another rather large bone, the sphenotic, this articulates below with the pro-otic. The pterotic and sphenotic together give rise to a large concave surface by which the hyomandibular articulates with the cranium. Several of the cranial nerves pass out through the bones of the auditory capsule. The ninth leaves by a foramen near the posterior border of the opisthotic, the fifth and seventh by a notch in the anterior border of the pro-otic.

A number of bones are likewise developed in connection with the orbit forming the orbital ring. Of these the most anterior, the lachrymal, is much the largest, the others are five to seven in number, the most ventral being the suborbital. The sclerotic coat of the eye is cartilaginous.

Two pairs of bones and one unpaired bone are developed in connection with the olfactory capsules, of these, the nasals are narrow bones lying next the lachrymals, but nearer the middle line; they overlap the second pair of bones, the irregular lateral ethmoids. These meet one another in the middle line, and are overlapped behind by the frontal. They articulate laterally with the lachrymal and palatine, and ventrally with the parasphenoid.

In a posterior view the foramen magnum and the four bones which surround it and together form the occipital segment are well seen. On the ventral side is the basi-occipital, terminated posteriorly by a slightly concave surface which articulates with the centrum of the first vertebra. The sides of the foramen magnum are formed by the exoccipitals, a pair of very irregular bones, pierced by a pair of prominent foramina for the exit of the tenth nerves. The exoccipitals also bear a pair of surfaces for articulation with corresponding ones on the neural arch of the first vertebra. The most dorsal of the four bones is the supra-occipital.

On the ventral surface of the cranium in front of the basi-occipital is seen the parasphenoid, a very long narrow bone which underlies the greater part of the cranium. Behind, it articulates dorsally with the basi-occipital and dorsolaterally with the pro-otics and opisthotics, in front it articulates dorsally with the lateral ethmoid and ventrally with the vomer. At the sides of the parasphenoid are the small alisphenoids articulating above with the postfrontals, in front with the frontals, and behind with the pro-otics.

The vomer is an unpaired bone lying immediately in front of the parasphenoid. In front it terminates with a thickened curved margin bearing several rows of small teeth; behind it tapers out into a long process which underlies the anterior part of the parasphenoid. Immediately dorsal to the vomer is another median bone, the median ethmoid; this is truncated in front and tapers out behind into a process which fits into a groove on the ventral side of the frontal.

Bones in connection with the upper jaw.

These bear a close resemblance to those of the Salmon. The most anterior bone is the premaxillae, a thick curved bone meeting its fellow in the middle line. The point of junction of the two is drawn out into a short process, and the oral surface is thickly covered with small teeth. The dorsal ends of the premaxillae are seen in the fresh skull to meet a large patch of cartilage. Behind the premaxillae is the maxilla, a long rod-like toothless bone, somewhat expanded at the upper end where it articulates with the premaxillae and vomer.

Articulating in front with the anterior end of the maxilla and with the lateral ethmoid is a very irregular bone, the palatine (fig. 12, 1); it articulates behind with two flat bones, the pterygoid and meso-pterygoid. The pterygoid is united behind with two more bones, the quadrate (fig. 12, 4) and meta-pterygoid. The quadrate is a rather stout irregular bone, bearing on its lower surface a prominent saddle-shaped articulating surface for the mandible. The palatine, pterygoid and quadrate bones are the ossified representatives of the palato-pterygo-quadrate bar of the Dogfish.