PETROMYZONTS OR LAMPREYS 455 



there are other serious difficulties against ranking Palceospondylus 

 with the Cyclostomes. 



(8) We cannot venture to say anything regarding the posi- 

 tion of a very puzzling set of extinct animals (Silurian and 

 Devonian) known as Ostracoderms or Hypostomata, but per- 

 haps they should be mentioned here. They were without 

 jaws, without a segmented axial skeleton in the trunk, without 

 any trace of girdles, and they had a complex dermal armature, 

 with a head shield. Examples: Pteraspis and Cephalaspis, 

 both without paired limbs ; and Pterichthys and Bothriolepis, 

 with strange armoured appendages fixed to the antero-lateral 

 angles of the body-shield. 



