XX ARTHRODIRA 5 
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un 
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 Red Sand- 
stone periods, and in North 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. 525). Two of the lateral plates are notched 
for the orbits, and between them there is an interorbital plate 
MC 
Le haa 
YELLS * 
LAGE 2248 
Fic. 323.—Restoration of Coccosteus decipiens. Old Red Sandstone of Scotland. x 1. 
A, Articulation of the cephalic and trunk shields; DB and DR, radials of the 
dorsal fin; 7, haemal arches and spines ; J/C, sensory canals; 4, neural arches 
and spines ; V7, notochord ; U, median plate; VB, basipterygium ; V2, 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. 
