260 



DIPNOI. 



t'la. 138.- 



-o, Ceratodus forsteri ; b, its pectoral fln (after Giinther). 

 plates (after Krefft). 



c, lower jaw with dental 



Arthrodira. 



The Arthrodira is a group of Devonian fishes showing some resem- 

 blance to and sometimes grouped with the Dipnoi. The vertebral column 

 appears to have been unossified, but the dorsal and ventral arches and 

 the fin-supports are weakly ossified. The head and anterior part of 

 the body are covered with large bony plates ; of wliich the head 

 plates are movably articulated with the anterior body plates. The 

 posterior part of the body appears to have been sometimes without 

 armour, sometimes with large dorsal and ventral plates ; the tail is hetero- 

 cercal, without scales. They were probably autostylic ; at any rate, no 

 trace of a hyomandibular has been observed. The chondrocranium was 

 probably unossified, but it is possible that in some genera there were 

 exoccipitals and ossified parachordals. The pectoral girdle and fin 

 have never been found, unless the slender hollow spines of Brachydeirus 

 are related to them. Teeth are absent, or confined to the pterygo-palatine 

 region, vomers, and lower jaw. Dorsal and anal fins are differentiated. 

 These fishes are very unlike anything now living, and must have had a 

 remarkable organisation. Coccosteus Ag., Dinichthys Newberry, including 

 forms of very large size (head-shield sometimes a metre across), Homosteus 

 Asmuss, Brachydeirus Koenen. 



Fia. 139. — Coccosteus decipiens, restored, J, showing pelvic fins, the heterocercal caudal fln 

 hypothetical (after Smith Woodward). 



Ostracodermi.* 

 Tlie fossils which are grouped together imder this heading have been 

 * Claypole, Pteraspidian fishes in the upper silurian rocks of N. America. 



