HYPOSTOMATA. 



507 



(4) "The rays which support the 

 caudal fin expansion, apparently 

 springing from the neural and haemal 

 arches, are dichotomised (at least the 

 neural ones), as are the corresponding 

 rods in the lamprey.'' 



Just behind the head lie two 

 small oblong plates (x., Fig. 242), 

 closely apposed to the commence- 

 ment of the vertebral column, one on 

 each side. The notochordal sheath 

 is calcified in the form of ring-shaped 

 or hollow vertebral centra with neural 

 arches. Towards the tail the arches 

 are produced into slender neural 

 spines, opposite which are shorter 

 h^mal ones. 



FIG. 242. - - Restored skeleton of 

 Palccospondylus gunni. After Tra- 

 quair. 



tl.c. , cirri of dorsal margin ; I.e. , long lateral 

 cirri; v.c., cirri of ventral margin; .. 

 nasal ring; /./., anterior trabeculo-pala- 

 tine part of cranium ; />., anterior depres- 

 sion or fenestra ; c., posterior depression 

 or fenestra; a., lobe divided off from 

 anterior part; /.., posterior or para- 

 chordal part of cranium; .r. , post-occi- 

 pital plates. 



Class HYPOSTOMATA or OSTRACODER.MI. 



Extinct forms without jaws, without a segmented axial skeleton in 

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

 with a head shield; Silurian and Devonian, e.g. Pteraspis-^n^. Cephalaspis, 

 both without paired limbs ; and Ptcrichthys and Bothriolepis, with 

 strange armoured appendages fixed to the antero-lateral angles of the 

 body-shield. 



FlG. 243. Ptcnchthys milleri. Lateral view. Restored by Traquair. 



