The head in all the species is covered with a great bony helmet. Behind this on the nape is another large shield, and between the two is usually a huge joint which Dr. Dean compares to the hinge of a spring-beetle (Elater).
As to the presence of limbs, no trace of pectoral fin or anterior limb has been found. Dean denies the existence of any structures corresponding to either limb, but Woodward figures a supposititious posterior limb in Coccosteus, finding traces of basal bones which may belong to it.
These monstrous creatures have been considered by Woodward and others as mailed Dipnoans, but their singular jaws are quite unlike those of the Dipneusti, and very remote from any structures seen in the ordinary fish. The turtle-like mandibles seem to be formed of dermal elements, in which there lies little homology to the jaws of a fish and not much more with the jaws of Dipnoan or shark.
The relations with the Ostracophores are certainly remote, though nothing else seems to be any nearer. They have no affinity with the true Ganoids, to which vaguely limited group many writers have attached them. Nor is there any sure foundation to the view adopted by Woodward, that they are to be considered as armored offshoots of the Dipnoans.
According to Dean we might as well refer the Arthrodires to the sharks as to the Dipnoans. Dean further observes ("Fishes Living and Fossil"):
Fig. 366.—Coccosteus cuspidatus Agassiz, restored. Lower Devonian. (After Traquair, per Woodward.)
"The puzzling characters of the Arthrodirans do not seem to be lessened by a more definite knowledge of their different forms. The tendency, as already noted, seems to be at present to regard the group provisionally as a widely modified offshoot of the primitive Dipnoans, basing this view upon their general structural characters, dermal plates, dentition, autostylism. But only in the latter regard could they have differed more widely from the primitive Elasmobranch or Teleostome, if it be admitted that in the matter of dermal structures they may be clearly separated from the Chimæroid. It certainly is difficult to believe that the articulation of the head of Arthrodirans could have been evolved after dermal bones had come to be formed, or that a Dipnoan could become so metamorphosed as to lose not only its body armoring, but its pectoral appendages as well. The size of the pectoral girdle is, of course, little proof that an anterior pair of fins must have existed, since this may well have been evolved in relation to the muscular supports of plastron, carapace, trunk, and head. The intermovement of the dental plates, seen especially in Dinichthys, is a further difficulty in accepting their direct descent from the Dipnoans."
Fig. 367.—Jaws of Dinichthys hertzeri Newberry. Upper Devonian. Ohio. (After Newberry.)