The Skeleton of the Fish 41 



in nearly all cases their homologies can be readily traced. In 

 the sharks and lampreys the skull constitutes a continuous 

 cartilaginous box without sutures. In the dipnoans and other 

 forms having a bony casque the superficial bones outside the 

 cranium may not correspond to the cartilaginous elements of 

 the soft skull itself. 



Bones of the Jaws. — The bones of the jaws are attached to 

 the cranium by membranes only, not by sutures, except in a 

 few peculiarly specialized forms. 



The Upper Jan'. — The preniaxillary (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_spinea . of -the premaxillary" are very long the upper jaw 

 may be thrust r."* fcr a considerable distance. The premaxil- 

 . \>ry is also olten knowii as in.tsrriiaxillary. 



Lying behind the premaxillary, its anterior end attached 

 within Jhe angle nf"*1ie premaxillary, is the"" maxillary (31), or 

 siiprainaxillary, 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 max- 

 illary 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. Al- 

 though often armed with teeth, these bones are to be considered 

 of the general nature of the membrane bones. In some de- 

 graded types of fishes (eels, morays, congers) the premaxillary 

 is indistinguishable, being united with the vomer and palatines. 



