528 



HYPOS TO MA TA. 



bral centra with neural arches. Towards the tail the arches are 

 produced into slender neural spines, opposite which are shorter 

 haemal ones. 



Class HYPOSTOMATA or OSTRACODERMI 



Extinct forms without jaws, without a segmented axial skeleton in 

 the trunk, without any trace of girdles, with complex dermal armature, 



FIG. 279. Pterichthys milleri. Lateral view.: Restored by Traquair. 



with a head shield ; Silurian" and Devonian, e.g. Pteraspis and Cepha- 

 laspts, both without paired limbs ; and Pterichthys and Bothriolepis, 

 with strange armoured paddles (probably not limbs in the ordinary 

 sense) fixed to the antero-lateral angles of the body-shield. Their 

 systematic position is very doubtful. They are the oldest known 

 Vertebrates. 



