2G0 



DIPNOI. 



Fia. 138. — a, Ceratodus forsteri ; b, its pectoral fin (after Giinther)." c, lower jaw with dental 



plates (after Kreft't). 



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 which the head 

 plates are movably articulated with the anterior body plates. The 

 posterior part of the body appears to have been sometimes without 

 aiinoiu", 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, vinless the slender hollow spines of Brachydeiriis 

 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 iinlike anything now living, and must have had a 

 remarkable organisation. Coccosteus Ag., Dinichthys Ncwberrj-, including 

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

 Asmuss, Brachydeirus Koenen. 



Fig. 139. — Coccosteus decipiens, restored, J, showing pelvic fins, the heterocercal caudal fin 

 hypothetical (after Smith Woodward). 



Ostracoderml.* 



The fossils which are grouped together under this heading have been 

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



