96 Arthrodira. 



Bristol, and from the Trias (Lettenkohle) of Wiirtemberg. 

 The characters of the skull of Ceratodus in the Museum of the 



FIG. 133. Phaneroplcuron . As.dersoni, Huxl. (restored by Dr. R. H. Traquair); Upper Old 

 Red Sandstone, Dura Den, Fife. 



Austrian Geological Survey, Vienna, suggest that the early 

 Mesozoic fish was geiierically distinct from the living fish 

 similarly named from the Queensland rivers. 



ORDER II. Arthrodira. 



Wall-case, The Coccosteus-like fishes have already been mentioned 



No. 4. (p. 76) as originally classified with the Ostracoderms in the 



un-natural and artificial group of " Placodermata." In them 

 the head and anterior portion of the trunk are armoured with 

 bony plates, and the head is movable with respect to the trunk. 

 In all the satisfactorily-known genera, there is an elaborately- 

 formed joint between the hinder angles of the head-shield and 

 a rounded process of the antero-lateral plates of the trunk ; an 

 arrangement unique among fishes and referred to in the name 

 ARTHRODIRA (joint-neck) now given to this group. The principal 

 upper teeth are fixed on the bones of the roof of the mouth ; the 

 lower jaw comprises only one bone on each side. The notochord 

 must have been persistent, and the paired fins are rudimentary 

 or absent. 



The Arthrodira are only provisionally placed among the 

 Dipnoi, on account of the very striking resemblance between 

 their dentition and that of certain mud-fishes, also because they 

 seem to have possessed a skull of the same type. 



Coccosteus (Fig. 134) is the best-known Arthrodiran, and 

 a fine series of specimens is exhibited from the Lower Old Bed 

 Sandstone of Scotland. There is also a unique head-shield 

 from the Upper Devonian of Scaumenac Bay, P.Q., Canada. 

 The eyes form notches in the head-shield ; there seem to be 

 premaxillae, and there is one large plate upon the cheek which 

 may be maxilla or suborbital, or both. The teeth are stout 

 and conical, in one close series on the mandible, clustered on the 

 palate. A pair of dermal plates occupies the position of clavicles ; 

 and the basal supports of the pelvic fins are often distinct (Fig. 

 1 34) . There is a membranous median dorsal fin, and the tail may 

 have been either diphy cereal or heterocercal. 



