528 HYPOSTOMATA. 
bral centra with neural arches. . Towards the tail the arches are 
produced into slender neural spines, opposite which are shorter 
heemal 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, 
Fic. 279.—Pterichthys miller?. Lateral view. —Restored by Traquair. 
with a head shield; Silurian’and Devonian, e.g. Pteraspzs and Cepha- 
laspis, both without paired limbs; and Pterichthys and Bothriolepi's, 
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, : 
