CHAPTER XXXIII 

 ARTHRODIRES 



>HE Arthrodires. Another large group of extinct fishes 

 mailed and helmeted is included under the general 

 name of Arthrodira* (ap&pos, joint; deipa, neck), or 

 Arthrognathi (apffpos, yvdOos, jaw), the latter term recently framed 

 by Dr. Dean with a somewhat broader application than the 

 former. 



These fishes differ from the Ostracophores, on the one hand, 

 in the possession of jaws and in the nature of their armored 

 covering. On the other hand, the nature of these jaws, the 

 lack of differentiation of the skeleton, and the uncertain charac- 

 ter of the limbs separate them still more widely from the true 

 fishes. Their place in the system is still unknown, but their 

 origin seems as likely to be traceable to Ostracophores as to 

 any other group. 



The head in all the species is covered with a great bony 

 helmet. Behind this on the nape is another large shield, and 



* "The name Arthrodira as given to Coccosteans, as distinguished from 

 the Antiarcha, is not altogether a satisfactory one, since at least from the 

 time of Pander the head of Pterichthys (Asterolepis) is known to be articu- 

 lated with the armoring of the trunk in a way closely resembling that of Coc- 

 costeus. This term may, however, be retained as a convenient one for the 

 order of Coccosteans, in which, together with other differentiating features, 

 this structure is prominently evolved. A renewed examination of the sub- 

 ject has caused me to incline strongly to the belief, as above expressed, that 

 Pterichthys and Coccosteans are not as widely separated in phylogeny as 

 Smith Woodward, for example, has maintained. But, as far as present evi- 

 dence goes, they appear to me certainly as distinct as fishes are from am- 

 phibia, or as reptiles are from birds or from mammals." (DEAN.) 



The name Placodermi used by McCoy in 1848 was applied to the Ostra- 

 cophores as well as to the Arthrodires. Hay revives it as the name of a super- 

 order to include the Antiarcha and the Arthrodira, the former being detached 

 from the Ostracophores. This superorder is equivalent to the subclass Azy- 

 gostei of Hay. 



