Ganoid scales are dimond-shaped scales found in lower order fishes such as the bichirs (Polypteridae), Bowfin (Amia calva), paddlefishes (Polyodontidae), gars (Lepisosteidae), and sturgeons (Acipenseridae).  Unlike ctenoid or cycloid scales, ganoid scales are comprised of bone.  They have a bony basal layer, a layer of dentin (also found in human teeth), and an outer layer of ganoine which is the inorganic bone salt for which these scales are named. 

Diamond-shaped ganoid scales form an interlocking protective armor for fish


 These scales interlock with peg-and-socket joints which make them quite inflexible, compared with ctenoid or cycloid scales, but very protective. (fishionary.fisheries.org/ganoid-scales)

Spotted Gar have ganoid scales