This section is from the book "A Manual Of British Vertebrate Animals", by Leonard Jenyns. Also available from Amazon: A Manual Of British Vertebrate Animals.
No true teeth; but the jaws covered with a lamellated substance resembling ivory, either entire, or divided in the middle by a suture: opercle small; branch-iostegous rays five on each side; both deeply concealed.
Jaws divided in the middle, so as to present the appearance of four teeth, two above and two below: skin rough, with small slightly projecting spines: body capable of inflation.
Jaws undivided: body very much compressed, short, truncated behind; rough, but without spines; not capable of inflation: dorsal and anal fins high and pointed, uniting with the caudal.
 
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