THE DEVONIAN PERIOD 



137 



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structure, and at the other with a head. The head had a circular 



mouth but no jaws. Its lack of jaws and paired fins cause it to 



rank below the true Fishes, 



and it is probably to be 



classed with the Lamprey 



Eels. 



Ostracoderms. — These 

 curious and bizarre forms 

 also represent a very simple 

 class of the Vertebrates. 

 For a long time they were 

 classed as simple Fishes, 

 but recent study has led 

 some to believe that they 

 are really transition forms 

 between the highest inver- 

 tebrates (Arthropods) and 

 the Fishes which rank very 

 low among the Vertebrates. 



A characteristic feature 

 is the cover or armor of 

 bony plates developed in the skin over the head and fore part 

 of the body, hence the name, which literally means " shell-skin." 

 The rear part of the body was generally covered with scales. Some 



•-.'•' 



Fig. 78 



Devonian Trilobites: a, Phacops logani 



(Hall); b, Homalonotus noticus (Clarke). 



Fig. 79 

 A very simple Devonian Vertebrate, Paleospondylus gunni. 

 (After Dean, restored by Traquair, from Chamberlin and 

 Salisbury's "Geology," courtesy of Henry Holt and Com- 

 pany.) 



had vertebrated tail fins and were fish-like in appearance (Fig. 80a), 

 while others looked much like Trilobites or King-crabs. Some had 

 a pair of jointed flappers or swimming paddles extending out from 



