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ANNUAL REPORT SMITHSONIAN INSTITUTION, 1934 



The last step in the adaptation to free-swimming is seen in a group 

 from the Upper Devonian, in which the connecting joint between the 

 head and body otherwise so characteristic of the Arthrodira has dis- 

 appeared, and the head and body shields have become fused (fig. 

 9, F). The peculiar connection between head and body in the 

 Arthrodira is practically unknown among fishes as it can only have 

 served to interfere with the function of swimming. In the bottom- 



FiGDRB 10. — A representative of the subclass Antiarchi (after Traquair). Pterichtys 

 from the English Middle Devonian : Upper left, from above ; upper right, from below ; 

 and lower figure, from the side. 



living Arthrodira the head-body joint has been retained until the 

 last Upper Devonian form, but in the more free-swimming repre- 

 sentatives the head loses its separate mobility. 



Thus we see how this group also, starting from typical bottom 

 forms, gradually conquered the ocean and learned to swim. 



The second group of the Placoderms (fig. 1) the so-called Antiarchi 

 (fig. 10) arc undoubtedly among the most peculiar beings which have 

 ever inhabited the sea. In their strongly encased head and trunk 

 they remind one a good deal of the Arthrodira. The head shield in 

 these also is connected with the trunk shield through a joint (fig. 10). 



