ARTHRODIRA 



535 



IV. Arthrodira. 



This group has been instituted for the reception of a number 

 of remarkable armoured Fishes of uncertain relationships which 

 flourished in Europe during the Devonian and Old Eed Sand- 

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

 Lower Carboniferous. The head {e.g. in Coccosteus) ^ 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 



Fia. 323. — 'Restoration ot Coccosteus decipiens. Old Red Sandstone of Scotland, x J. 

 A, Articulation of the cephalic and trunk shields ; DB and DR, radials of the 

 dorsal fin ; H, haemal arches and spines ; MC, sensory canals ; N, neural arches 

 and spines ; NT, notoohord ; U, median plate ; VB, basipterygium ; VR, 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 Binichthys, 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. 



