The Upper Jaw.—The premaxillary (32) lies on either side and forms the front of the upper jaw. Its upper posterior tip or premaxillary spine projects backward almost at right angles with the rest of the bone into a groove on the ethmoid. There is often a fold in the skin by which this bone may be thrust out or protracted, as though drawn out of a sheath. When the spines of the premaxillary are very long the upper jaw may be thrust out for a considerable distance. The premaxillary is also often known as intermaxillary.
Lying behind the premaxillary, its anterior end attached within the angle of the premaxillary, is the maxillary (31), or supramaxillary, a flattened bone with expanded posterior tip. In the striped bass this bone is without teeth, but in many less specialized forms, as the salmon, it is provided with teeth and joined to the premaxillary in a different fashion. In any case its position readily distinguishes it. In some cases the maxillary is divided by one or more sutures, setting off from it one or more extra maxillary (supplemental maxillary) bones. This suture is absent in the striped bass, but distinct in the black bass, and more than one suture is found in the shad and herring. The roof of the mouth above is formed by a number of bones, which, as they often possess teeth, may be considered with the jaws. These are the palatine bones (21), one on either side flanking the vomer, the pterygoid (20), behind it and articulating with it, the mesopterygoid (22), on the roof of the mouth toward the median line, and the metapterygoid (23), lying behind this. Although often armed with teeth, these bones are to be considered of the general nature of the membrane bones. In some degraded types of fishes (eels, morays, congers) the premaxillary is indistinguishable, being united with the vomer and palatines.
Fig. 26.—Roccus lineatus. Face-bones, shoulder and pelvic girdles, and hyoid arch.
- 17. Hyomandibular.
- 18. Symplectic.
- 19. Quadrate.
- 20. Pterygoid.
- 21. Palatine.
- 22. Mesopterygoid.
- 23. Metapterygoid.
- 24. Preopercle.
- 25. Opercle.
- 26. Subopercle.
- 27. Interopercle.
- 28. Articular.
- 29. Angular.
- 30. Dentary.
- 31. Maxillary.
- 32. Premaxillary.
- 33. Interhyal.
- 34. Epihyal.
- 35. Ceratohyal
- 36. Basihyal.
- 37. Glossohyal.
- 38. Urohyal.
- 39. Branchiostegal.
- 49. Preorbital.
- 50. Suborbital.
- 51. Nasal.
- 52. Supratemporal.
- 53. Post-temporal.
- 54. Supraclavicle.
- 55. Clavicle.
- 56. Postclavicle.
- 57. Hypercoracoid.
- 58. Hypocoracoid.
- 60. Actinosts.
- 61. Pectoral fin.
- 62. Pelvic girdle.
- 63. Ventral fin.
The upper jaw of the shark is formed from the anterior portion of the palatine bones, which are not separate from the quadrate, the whole forming the palato-quadrate apparatus. In the himæra and the dipnoans this apparatus is solidly united with the cranium. In these fishes the true upper jaw, formed of maxillary and premaxillary, is wanting.
Fig. 27.—Lower jaw of Amia calva (Linnæus), showing the gular plate.