AHTHRODIRA. 



65 



supports of the median fin are equal in number to the apposed 

 arches of the endoskeleton. The paired fins are rudimentary. 



In the Coccosteidae the head-shield consists of few plates, 

 and is sometimes notched for the orbits, sometimes encircles 

 them. The narial openings are seen near the front border of 

 the shield. The maxilla and premaxilla are well-developed but 

 toothless. The dentition, when present, consists of conical teeth 

 fused with the oral margin of the mandible and with two inner 

 pairs of bones in the upper jaw (presumably palatine and 

 vomerine). The abdominal region is armoured both dorsally 

 and ventrally, the large dorsal plate having a deep inner 

 longitudinal keel, evidently for connection with the neural 

 arches of the endoskeletal axis. 



FIG. 50. 



Coccosteus decipiens ; restoration of cranial and dorsal shield by R. H. Traquair, 

 about one-third nat. size. L. Old Red Sandstone; N. Scotland, a.d.l., 

 anterior dorso-lateral ; a.l., anterior lateral ; c, central ; <, ethmoid ; 

 e.o., external occipital ; in, marginal ; m.d., median dorsal ; m.o., median 

 . occipital ; mx., maxillo-suborbital ; n, narial opening ; o, orbit ; p, pineal ; 

 p.d.l., posterior dorso-lateral; p.L, posterior lateral; p.mx., premaxilla; 

 P.O., preorbital ; pt.o., postorbital ; x, operculum (?). The dotted lines 

 indicate the course of the mucus-canals. 



w. 5 



