SUB-CLASS III 



ARTHRODIKA 



59 



processes for anion with the head shield. Ventral body armowr comprising two or 

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

 I'hites more or less tuberculated. Dorsal fin small ; anal fin unknown; tail diphy- 

 cercal or heterocercal. Devonian. 



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

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

 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 



osteus decipv 



Fig. 110. 



Ag. Left antero-dorso-lateral plate of trunk, inner (A), anil outer i 

 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 : 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, 1/2- 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, I .S.A. 



Brachydirus. v. Koenen. Considered bv von Koenen to differ from Cocco- 

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

 pectoral fins. Upper Devonian ; Bicken, Xassau. 



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

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



Chelyophorus, Ag. Upper Devonian ; Russia. 



Dinichthys, Xewberry (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 with 

 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 Coccostmis. Pre- 



