HISTORICAL GEOLOGY 



tracoderms (literally "shell skin "), which were not fishes at all, 

 in the strict sense (Fig. 380). It is not cer- 

 tain that they possessed jaws, but if they 

 did, there is some reason to think that the 

 jaws worked horizontally as in beetles. 

 Strong resemblances to some of the early 

 arthropods are seen in their bony head 

 shields and in the closely spaced eyes. In 

 fact, their claim to a place among the 

 vertebrates rests chiefly on the possession 

 of a tail fin which seems to imply that 

 they had a rudimentary spinal column. 

 Since none are now in existence it is hard 

 to determine their real character. 



The true fishes, which are furnished 

 with jaws of the customary type and one 

 or two pairs of fins along their flanks, 

 are represented in the Devonian fauna by many strange and 

 some very large species. Some, on the other hand, were not 



FIG. 379. A Devo- 

 nian coral, showing 

 the cup with radiat- 

 ing partitions. 



FIG. 380. An ostracoderm. 



so unlike those of to-day that the untrained eye would readily 



note the difference. Others had the head cased in heavy 



plates of bone, with only the rear part of the body left in 



a flexible condition. In 



modern fishes the limb 



bones do not extend out 



into the fins, but end in 



blunt plates to which the 



fin rays are attached (Fig. FlG 381 _ Tail of a primitive ^ 



383). The Devonian fishes, fringe above and below. 



