PALJLOSPONDYLUS GUNNI. 



419 



springing from the neural and hzemal 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. 202), «/■«• 



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 aiches 

 are produced into slender neural 



spines, opposite 

 htemal ones. 



which are shorter 



Between Cyclostomata and Fishes, 

 some authorities would rank a num- 

 ber of fossil types without jaws, 

 Cope's Ostracodermi, e.g. Pleraspis, 

 Cephalaspis, Pterichthys. 



Fig. 202. — Restored skeleton of 

 Palaospondylus. — After Traquair. 



d.c.y cirri of dorsal margin ; I.e., long_ lateral 

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

 nasal ring ; t.p., anterior trabeculo-pala- 

 tine part of cranium ; b., anterior depres- 

 sion or fenestra ; c, posterior depression 

 or fenestra; a., lobe divided off from 

 anterior part; p. a., posterior or para- 

 chordal part of cranium; x. t post-occi- 

 pital plates. 



