SUB -CLASS III 



ARTHRODIRA 



59 



processes for union toith the head shield. Ventral body armour comprising tvjo or 

 three pairs of lateral plates and two small unpaired median plates. All external 

 plates more or less tuberculafed. Dorsal fin small ; anal fin unknown ; tail dip)hy- 

 cercal or heterocercal. Devonian. 



Coccosteus, Ag. (Liognathus, Newb.), (Figs. 110, 111). Head and trunk 

 broad, the dermal plates covered with stellate tubercles. Cranial plates united 

 by sutures, not fused. Orbit forming a notch in the anterior part of the 

 lateral border of the shield ; a bony sclerotic ring present. Besides the 

 "maxilla" and "premaxilla," one or two inner pairs of dentigerous bones also 



Coccosteus decipiens, Ag. Left antero-dorso-lateral plate of trunk, inner {A), and outer (B), aspects. 

 Lower Old Red Sandstone ; Scotland. 



present in the upper jaw. In front of the ventral body shield occur a pair 

 of transversely elongated slender plates, perhaps representing the pectoral 

 arch j a small pectoral spine, but never any traces of pectoral fins observed. 

 Ribs absent. Dorsal fin short-based. Tail destitute of scales. Nearly com- 

 plete but imperfectly preserved skeletons of C. decipiens, Ag. (Figs. 110, 111), 

 occur in the Lower Old Red Sandstone of Scotland. Fragments of other 



Fig. 111. 

 Coccosteus decipiens, Ag. Restoration, i/o. Lower Old Red Sandstone ; Scotland (after Smith Woodward). 



species in the Upper Devonian of England, Ireland, Germany, Russia, and 

 Canada. Also from Corniferous Limestone (Middle Devonian), Ohio, U.S.A. 



Brachydirus, v. Koenen. Considered by von Koenen to differ from Cocco- 

 steus in the presence of a pair of hollow spines which perhaps represent 

 pectoral fins. Upper Devonian ; Bicken, Nassau. 



Phlycfaenaspis, Traquair. Lower Devonian ; Canada, west of England, and 

 Russian Poland. Acanthaspis, Newb. Lower and Middle Devonian ; U.S.A. 



Chelyophorus, Ag. Upper Devonian ; Russia. 



Dinichthys, Newberry (Fig. 112). Very large, the head sometimes 

 measuring 1 m. in length and 0"7 m. in breadth. Median occipital plate sub- 

 triangular with prominent median process behind ; lateral occipital plates Avith 

 very deep articular sockets for the reception of the process on the antero-dorso- 

 lateral plates of the body shield. Orbit forming a notch as in Coccosteus. Pre- 



