Heterostraci. (A) Schematic illustration of the integumentary skeleton of a generalized early Devonian heterostracan (e.g. Poraspis or Pteraspis). The lamellar, acellular bony plate (isopedin) is surmounted by a layer of cancellous bone and ornamented by odontode-like tubercles composed of dentine capped with enameloid. (B) SEM section of the superficial region of a portion of the integumentary skeleton of Tesseraspis demonstrating the enameloid-dentine junction (arrow). (C,D) SEM sections of the integumentary skeleton of Anglaspis. Scale bars: B = 40 µm; C = 210 µm; D = 80 µm.