 The difference between teleosts and other bony fish lies mainly in their jaw bones ; teleosts have a movable premaxilla and corresponding modifications in the jaw musculature which make it possible for them to protrude their jaws outwards from the mouth . This is of great advantage , enabling them to grab prey and draw it into the mouth . In more derived teleosts , the enlarged premaxilla is the main tooth @-@ bearing bone and the maxilla , which is attached to the lower jaw , acts as a lever , pushing and pulling the premaxilla as the mouth is opened and closed . Other bones further back in the mouth serve to grind and swallow food . Another difference is that the upper and lower lobes of the tail ( caudal ) fin are about equal in size . The spine ends at the caudal peduncle , distinguishing this group from other fish in which the spine extends into the upper lobe of the tail fin . 
