The Structure of Dinichthys 



203 



This compendium alone shows that the question of the relationship of the Arthrodires 

 is a difficult one. The facts we know are so few that the subjective opinions of the in- 

 vestigators play too great a part in their conclusions. In the following sections I shall try 

 to analyze all the different relationships of the Arthrodira. 



Text-figure 90. 

 A total reconstruction of Dinichthys showing the fish in action. 



THE RELATIONS BETWEEN THE ARTHRODIRA 

 AND OTHER FISH GROUPS 



When we know that the Arthrodira had no Dipnoi-like teeth, that the mechanism 

 of their jaws was unusual and that they were not autostylic, their relation to the Dipnoi, 

 based chiefly on these characters, must be regarded as improbable. Furthermore, they 

 have so little in common with the Holocephali that it is difficult to find any relation- 

 ship there. 



The opinion of Jaekel that the Arthrodira are closely related to the Chondrostei, 

 is based on the great likeness of the shoulder girdle of Rhynchodonts from Wildungen 

 to that of the Acipenseridae, and to the body carapace of the Arthrodira. The likeness 

 between Rhynchodontus and Aapenser is indeed quite great. Apart from the analogies 

 in shoulder girdle structure as discussed by Jaekel, they also show close relationships in 

 the structures of the neurocranium, head roof, gills and fins. On the other hand, it is 



