ARTHRODIRA 535 



IV. Arthrodira. 



This group has been instituted for the reception of a number 

 of remarkable armoured Fishes of uncertain relationships which 

 nourished in Europe during the Devonian and Old Red Sand- 

 stone periods, and in Xorth America from the Devonian to the 

 Lower Carboniferous. The head (e.g. in Coccosteus) l is invested 

 dorsally by a series of median and lateral symmetrically-disposed 

 tuberculated plates (Fig. 323). Two of the lateral plates are notched 

 for the orbits, and between them there is an interorbital plate 



FIG. 323. Restoration of Coccosteiis decipiens. Old Red Sandstone of Scotland. x\. 

 A, Articulation of the cephalic and trunk shields ; />/>' and T)R, radials of the 

 dorsal fin ; //, haemal arches and spines ; MC, sensory canals ; 3", neural arches 

 and spines ; NT, notochord ; U, median plate ; VB, basipterygium ; VII, radialia 

 of the pelvic fin. (From Parker and Haswell, after Bashford Dean and Smith 

 Woodward. ) 



which either has a pit on its inner surface or is perforated by an 

 open funnel, as in Dinichthys, possibly for a parietal or a pineal 

 organ. Some of the bones present some analogy, to say the least, 

 to certain of the dermal bones of a typical Teleostome, apparently 

 representing such elements as paired parietals and frontals, a 

 dermal mesethmoid, and toothless premaxillae and maxillae (Fig. 

 324, A). As in the Antiarchi, the anterior portion of the 

 trunk is also armoured, above by a dorsal shield, formed by 

 median and lateral plates, and below by a similarly constructed 

 ventral shield (Fig. 324, B). A huge joint connects the head 

 and trunk shields : hence the term Arthrodira or " joint-neck." 

 The rest of the body is naked. 



Pectoral fins are unknown, but pelvic fins, each supported by 

 a stout basal plate or basipterygium, and with traces of radials, 

 are present. There is a small dorsal fin. Little is known of 

 the primary cranium, but in the trunk and tail it is evident 



1 Traquair, Ann. Nat. Hist. (6), v. 1890, p. 125. 



