vi. 9 PLACODERMS 189 



jaws were present and that probably the hyomandibula was not sus- 

 pensory. He therefore classed the fish with the placoderms, in spite 

 of the absence of any dermal skeleton. So far as can be discovered, 

 all these placoderm fishes except the acanthodians were specialized 

 types and have not left any later descendants. Indeed it may well be 

 that they have been preserved only because of the great extent of their 

 armour; less heavily protected relatives may have existed but have 

 not survived as fossils. The remains that are known are sufficient to 

 establish the fact that there were, in the Devonian period, numerous 

 types of fish possessing a bony skeleton. 



